Model Output Discussion October 2012

[damianslaw]

Reliable timeframe - after a couple more showery days, a wet 36 hours or so is on the cards for central parts thanks to frontal activity associated with lower heights to our SW stemming from ex hurricane Nadine. The weekend though will see a brief ridge - very timely so a good dry weekend could be on the cards with probably mist and fog slow to clear some parts and possible frost in places.

Into next week - the remains of Nadine will bring a bout of wet and windy weather - possibly very wet weather, so something to watch, lets hope it doesn't become a slow moving feature again. The models do indicate a very strong jet which should kick the system away to the east.

Longer term - both ECM and GFS are painting a very disturbed autumnal scene with deep low pressure systems and a jet becoming more NW-SE aligned as heights build both to the NW and NE, which would result in a rather chilly feel with snow for the highest ground and temps struggling to reach double figures in northern parts. October 2008 and 2010 both saw a blast of polar air during the second half... and there are strong hints Oct 2012 will see the same..

[Nick F]

Looks like the second child of Nadine could be arriving across the southern half of the UK on Friday, in the form of a wave moving east along a frontal zone which loiters close to southern England tomorrow onwards. Nadine soon to become ex-tropical still wallows north of the Azores later this week, and injects some warm/moist Tm air from the west beneath the frontal zone across southern Britain later this week.

Steep thermal gradient shown on theta-e/w charts along boundary/baroclinic zone, so some more large totals possible - particularly across SW England and Wales - where a threat of flooding could arise from what could be a rather wet 36hr period later this week.

[Nick B]

Yes FI, But the GFS has been hinting this a few times over the last week. Something to keep an eye on with -3 and -4 uppers over the uk. And the Jet way South of the UK.

Without wishing to put a dampener on this, for those unsure about charts like this, even if they did verify as shown they would be very unlikely to produce anything more interesting than cold rain or sleet for anywhere other than high ground (300m+). Those who've seen charts like this come and go a few times will know that snow wouldn't quite make it because...

1. Temperature at ground level would be unlikely to be quite low enough yet to really deliver.

2. Dewpoints would be very unlikely to stay low enough.

3. Although the 850s shown are a few degrees below zero, it's marginal enough that it really would need to have been sourced from a cold continent in order not to be modified by the time it reached here. Look where the cold(er) air is sourced from... is it genuinely cold enough?

4. It is quite possible that if precipitation were in heavy enough bursts, there could be areas where wet snow fell for short periods of time, but it would be unlikely to stick around for long.

5. A few other factors mentioned in John Holmes' thread/post (I think it might have been called 'What to look for to get snow', but I may be wrong).

I stress 'highly unlikely', not impossible, meaning any fairly excitable new followers to these charts may not want to let their hopes be raised too much just yet, based on these charts... unless even more favourable ones come into a much closer timeframe, say 72 hours or probably even less. It is still rather early even for an unusually early winter appearance mid-month. End-October we all know can deliver a pleasing spectacle (2008!) but for snow which will stick more than a day, I wouldn't be looking really seriously until perhaps November. I'll be pleased to be proved wrong by events!

Seeing more and more of these charts appear and with colder uppers may be taken as positive signs, but I feel we have a fair bit more autumnal weather to get through first, which is in itself interesting for some.

Edited October 2, 2012 by Nick B

[Polar Maritime]

Without wishing to put a dampener on this, for those unsure about charts like this, even if they did verify as shown they would be very unlikely to produce anything more interesting than cold rain or sleet for anywhere other than high ground (300m+). Those who've seen charts like this come and go a few times will know that snow wouldn't quite make it because...

1. Temperature at ground level would be unlikely to be quite low enough yet to really deliver.

2. Dewpoints would be very unlikely to stay low enough.

3. Although the 850s shown are a few degrees below zero, it's marginal enough that it really would need to have been sourced from a cold continent in order not to be modified by the time it reached here. Look where the cold(er) air is sourced from... is it genuinely cold enough?

4. It is quite possible that if precipitation were in heavy enough bursts, there could be areas where wet snow fell for short periods of time, but it would be unlikely to stick around for long.

5. A few other factors mentioned in John Holmes' thread/post (I think it might have been called 'What to look for to get snow', but I may be wrong).

I stress 'highly unlikely', not impossible, meaning any fairly excitable new followers to these charts may not want to let their hopes be raised too much just yet, based on these charts... unless even more favourable ones come into a much closer timeframe, say 72 hours or probably even less. It is still rather early even for an unusually early winter appearance mid-month. End-October we all know can deliver a pleasing spectacle (2008!) but for snow which will stick more than a day, I wouldn't be looking really seriously until perhaps November. I'll be pleased to be proved wrong by events!

Seeing more and more of these charts appear and with colder uppers may be taken as positive signs, but I feel we have a fair bit more autumnal weather to get through first, which is in itself interesting for some.

It was only the temperatures I was quoting in my post.

But I agree snow is highly unlikely IF these chart's verified, except for the very high ground in the North.

[syed2878]

hi people. today's discussion from noaa.

PRELIMINARY EXTENDED FORECAST DISCUSSION

943 AM EDT TUE OCT 02 2012

VALID 12Z SAT OCT 06 2012 - 12Z TUE OCT 09 2012

...COLD SHOT TO INVADE THE CENTRAL AND EASTERN STATES...

GENERAL FLOW PATTERN

======================================

THE GUIDANCE IS IN GENERAL AGREEMENT THAT A HIGH LATITUDE REX

BLOCK... CLOSED HIGH ATOP CLOSED LOW... IN THE NORTHEAST PACIFIC

WILL REMAIN QUASI-STATIONARY THROUGH THE MEDIUM RANGE. THIS IS A

CHANGE IN THE GUIDANCE SINCE 24 HRS AGO WHICH PREVIOUSLY

PROGRESSED THE BLOCK AS A WHOLE INTO WESTERN NORTH AMERICA. THE

RESULT SHOULD KEEP BROAD TROUGHING DOWNSTREAM THROUGH THE

CENTRAL/EASTERN CONUS CENTERED ALONG 90W AS THE WESTERN ATLANTIC

RIDGE NEAR BERMUDA WEAKENS. ALSO KEEPING THE PATTERN IN PLACE IS

TROUGHING NORTH OF HAWAII REINFORCED BY THE EXPECTED EXTRATROPICAL

TRANSITION OF TC MALIKSI OUT OF THE W PAC EARLY NEXT WEEK.

　

MODEL PREFERENCES

=================

OUTSIDE THE 00Z CANADIAN... THE DETERMINISTIC AND ENSEMBLE

GUIDANCE WERE IN BETTER THAN AVERAGE AGREEMENT. THE PRESSURES WERE

BASED ON A 50/50 BLEND OF THE 00Z UKMET/00Z ECMWF ON FRI/D3... A

40/30/30 BLEND OF THE 00Z UKMET/00Z ECMWF/00Z GFS ON

SAT-SUN/D4-5... AND A 50/50 BLEND OF THE 00Z GFS/00Z ECMWF

MON-TUE/D6-7. THE ECMWF AND GFS ENSEMBLE MEANS SHOWED VERY GOOD

AGREEMENT WITH THEIR OPERATIONAL RUNS AND EACH OTHER. WHEN

COMPARED TO CONTINUITY, THIS ALLOWS THE POLAR FRONT TO MAKE

FURTHER/QUICKER INROADS THROUGH THE INTERMOUNTAIN WEST INTO THE

SOUTHWEST AND MEXICO... AND CAUSES THE CLIPPER SYSTEM SCRAPING THE

US/CANADIAN BORDER LATE IN THE PERIOD TAKE A MORE ARCING TRACK

THROUGH WESTERN CANADA THAN CENTRAL CANADA AS A SLIGHTLY WARMER

SYSTEM. BY TUE/D7... THE ECMWF IS A BIT QUICKER TO BRING THE UPPER

LOW INTO CALIFORNIA THAN THE GFS WHICH IS A REVERSAL OF THEIR

RESPECTIVE TYPICAL BIASES... SO A COMPROMISE SOLUTION SEEMS

PRUDENT.

　

WEATHER IMPACTS

===============

THE AMPLITUDE OF THE MID-CONTINENT TROUGH IS EXPECTED TO KEEP

CONDITIONS COOL OVER MUCH OF THE LOWER 48 AS STANDARDIZED

ANOMALIES WITHIN THE 500 HPA HEIGHT AND 850 HPA TEMPERATURE FIELDS

ARE EXPECTED TO BE 2-3 SIGMAS BELOW AVERAGE FOR THE FIRST WEEK OF

OCTOBER... PARTICULARLY ACROSS THE PLAINS/MIDWEST/MID-MISSISSIPPI

VALLEY. THIS COULD RESULT IN TEMPERATURES AS MUCH AS 20-25 DEGREES

BELOW NORMAL FROM THE CENTRAL/SOUTHERN PLAINS INTO TEXAS. LAKE

EFFECT PRECIPITATION /PROBABLY A CHILLY RAIN/ IS ANTICIPATED

ACROSS THE GREAT LAKES THIS WEEKEND ON THE SOUTHWEST SIDE OF A

DEPARTING DEEP CYCLONE. A SHORTWAVE MOVING DOWN THROUGH THE

NORTHERN ROCKIES AND INTO THE PLAINS INCREASES UPSLOPE RAIN/SNOW

POTENTIAL ACROSS WYOMING/NORTHERN COLORADO/NEBRASKA ON FRIDAY...

WHICH WILL QUICKLY SPREAD EASTWARD AS RAIN ACROSS THE

MID-MISSISSIPPI AND OHIO VALLEYS INTO THE MID-ATLANTIC/NEW ENGLAND

STATES THIS WEEKEND. A POSSIBLE EARLY-SEASON CLIPPER SYSTEM MOVING

OUT OF WESTERN CANADA COULD BRING A SECOND CHANCE FOR RAIN OR SNOW

TO THE IMMEDIATE CENTRAL US/CANADIAN BORDER LATE SUNDAY AND

MONDAY. MEANWHILE... MODERATE RAINS SHOULD PERSIST OVER THE

FLORIDA PENINSULA THROUGH AT LEAST SUNDAY OR POSSIBLY INTO NEXT

TUESDAY AS A SUCCESSION OF FRONTAL BOUNDARIES TAP INTO MOISTURE

FROM THE WESTERN CARIBBEAN AND GULF OF MEXICO. THE CLOSED LOW

MOVING INTO CALIFORNIA COULD LEAD TO UPSLOPE PRECIPITATION INTO

THE SISKIYOU AND SIERRA NEVADA RANGES SUNDAY INTO NEXT TUESDAY,

BUT ITS MAIN IMPACT SHOULD BE A SLIGHT COOLDOWN OF CALIFORNIA AND

THE DESERTS EARLY NEXT WEEK.

　

ROTH/FRACASSO.

http://www.hpc.ncep.noaa.gov/preepd/preepd.html

[Nick B]

It was only the temperatures I was quoting in my post.

But I agree snow is highly unlikely IF these chart's verified, except for the very high ground in the North.

Quite right... I was fairly certain you would know this! Still good to see trends like this coming along more often.

[davehsug]

Without wishing to put a dampener on this, for those unsure about charts like this, even if they did verify as shown they would be very unlikely to produce anything more interesting than cold rain or sleet for anywhere other than high ground (300m+). Those who've seen charts like this come and go a few times will know that snow wouldn't quite make it because...

1. Temperature at ground level would be unlikely to be quite low enough yet to really deliver.

2. Dewpoints would be very unlikely to stay low enough.

3. Although the 850s shown are a few degrees below zero, it's marginal enough that it really would need to have been sourced from a cold continent in order not to be modified by the time it reached here. Look where the cold(er) air is sourced from... is it genuinely cold enough?

4. It is quite possible that if precipitation were in heavy enough bursts, there could be areas where wet snow fell for short periods of time, but it would be unlikely to stick around for long.

5. A few other factors mentioned in John Holmes' thread/post (I think it might have been called 'What to look for to get snow', but I may be wrong).

I stress 'highly unlikely', not impossible, meaning any fairly excitable new followers to these charts may not want to let their hopes be raised too much just yet, based on these charts... unless even more favourable ones come into a much closer timeframe, say 72 hours or probably even less. It is still rather early even for an unusually early winter appearance mid-month. End-October we all know can deliver a pleasing spectacle (2008!) but for snow which will stick more than a day, I wouldn't be looking really seriously until perhaps November. I'll be pleased to be proved wrong by events!

Seeing more and more of these charts appear and with colder uppers may be taken as positive signs, but I feel we have a fair bit more autumnal weather to get through first, which is in itself interesting for some.

but not all. Autumn also consists of some quiter drier warmer spells too! Sadly missing, possibly looking for the lost summer again!

[IDO]

The GFS Ens still has HP near the UK for most of the 06z run, although LP is more influential. The Op run reflects this, giving a changeable picture. Looks like average temps through the run but feeling cooler when the wind comes from the north.

Lots of rain is the main headline for this 15 days but no real indication that the jetstream is going to park to our south; the opposite for the majority, on this run.

Way at the end of FI HP dominates the Atlantic, so a mobile westerly pattern possible?

GFS look to be unsure past T144 but it does look like the end of any Indian Summer hopes, with Autumnal or cooler being the main protagonists.

Edited October 3, 2012 by i'm dreaming of...

[Gibby]

Good morning. Here is the morning look at the 00zs from GFS, UKMO and ECM.

All models show a very complex system over the next few days. In basic terms a showery west flow covers Britain but a layer of warm tropical maritime air lies to the SW of Britain spawned from the remains of ex tropical storm Nadine. A spore of this will move East across Southern Britain, most likely over Friday before it gets swept away SE on Saturday as a timely ridge of High pressure settles things down somewhat for 24-36hrs over the weekend.

GFS then lies a bit on its own this morning holding a ridge over Northern Britain to start next week while Southern Britain is attacked once more from Low pressure to the SW. Although the offending Low pressure weakens it carries rainfall NE over Britain in the following 24 hours opening the door to the NW for the Atlantic to bring further Low pressure in to affect all areas later in the week. In FI this morning a very changeable and often chilly spell seems likely with rain or showers at times. The showers may fall as snow at times on Scottish mountains. However, in general the run is not as deeply cold and disturbed as last night as High pressure ridges in periodically from the SW to give some drier and brighter spells at times especially in the South.

The GFS Ensembles continue to programme a wet spell for southernmost locations later this week with a changeable pattern thereafter. 850's dip below normal with thine and never meaningfully rise above the long term average. For Northern areas a typical Atlantic spell occurs with rain at times and uppers which stay close to the long term mean throughout though many members show a sine wave pattern as cold and warm sectors in association with depressions cross by.

The Jet Stream continues to blow across the UK from the West before splitting into two arms next week with the Northern arm dominating and ridging up to a position North of the UK then sinking South through next week.

UKMO for midnight shows Low pressure over Wales with an unsettled day likely with rain or showers under cyclonic and rather cool breezes.

ECM maintaines the general theme of last night with Low pressure becoming deep and complex near Northern Britain with rain, strong winds and heavy rain at times all features of the weather in the later stages of the run. Some brief and drier interludes are possible too in rather cool conditions.

In Summary apart from the GFS operational which seems to have its own way of thinking with regard to early next week the general trend (although there are differences in the specific positioning of Low pressure) shared by the Euro's are for Low pressure to migrate close to the NW for the early stages and becoming complex for the rest of the week. Showers or spells of rain are likely for all in temperatures a smidgen below normal at the surface emphasised by the wind. Before all this happens there looks like a window of decent weather though at the weekend.

[johnholmes]

thanks Gibby; certainly anyone expecting anything but unsettled and on the cold side in the medium term, 6-15 days ahead would be going against this pair of 500mb anomaly charts. The NOAA issue last evening was not that different either.

http://web1.met.psu.edu/~fxg1/ECMWF_0z/hgtcomp.html

[Cloud 10]

thanks Gibby; certainly anyone expecting anything but unsettled and on the cold side in the medium term, 6-15 days ahead would be going against this pair of 500mb anomaly charts. The NOAA issue last evening was not that different either.

http://web1.met.psu....0z/hgtcomp.html

That atlantic high anomaly is quite persistent at the moment.

The anomaly chart for September just gone is fairly similar as well.

[IDO]

thanks Gibby; certainly anyone expecting anything but unsettled and on the cold side in the medium term, 6-15 days ahead would be going against this pair of 500mb anomaly charts. The NOAA issue last evening was not that different either.

http://web1.met.psu....0z/hgtcomp.html

I am looking at GFS for the last couple of runs and averaging out the temp highs (at 2m), for the 6-15 day period, and they are mostly around average or above (records say 14.5 October av. high Birmingham). Also the Op runs and ENS all have a high close by, or encroaching on the UK (GFS very consistent to date, but may change). So I am not sure the 6-15 day period is currently being progged as cold. Could you define "cold side", what sort of temps. do you mean? The GFS to me is signalling rain as our main concern, not cold (although there may be some 36-48 hours of cooler weather via a trough in the middle of that period).

Certainly ECMW have a cooler option for the period but I await GFS to follow suit before I would be making that bold statement (those anomaly charts are so tiny that the finer detail of a UK map can sometimes be more precise, especially when we are on the cusp of lower heights).

To me the GFS is just average October fare. Hey, but let us know if I am reading GFS wrong.

[johnholmes]

Not ignoring your comment but just off out I'll attempt to answer later this evening.

[Gibby]

Good evening. Here's a look at the 12zs from GFS, UKMO and ECM tonight.

All models are clear on a brief respite taking shape tomorrow as a very weak ridge crosses through the day. With isobars spacing out too winds will be lighter than today with sunny spells. Late tomorrow a warm frontal wave approaches the SW with cloud and then rain following spreading to Southern England and Wales overnight and through Friday. further North the light weakly showery airflow will persist. Saturday sees the frontal wave clear away SE leaving a decent day for Saturday as a ridge passes over all areas, On Sunday low pressure to the SW pulls frontal troughs back NE over Southwestern areas and continuing on NE to all areas over Monday.

GFS then shows the Low clearing away East on tuesday with a short-lived ridge collapsing SE over Britain with a drier day replacing the rains of Monday. The midweek period shows the drier interlude continuing over the South while Northern areas become wet again in association with a Low pressure passing East to the North. In FI tonight GFS continues to collapse Low pressure SE down the Eastern side of the UK with High pressure out to the SW, fairly close at times.The weather would see rain and showers for all with some drier interludes too, especially in Southern areas. Temperatures at the surface would not be too far from Normal apart from the short Northerly spell mid run.

The GFS Ensembles contine to show a changeable pattern. A lot of rain is shown for the South between the 5th and the 9th with some in the North too through the run. Temperatures at the 850 level will alternate either side of the long term mean level and this will reflect in never far from normal temperatures at the surface under this Atlantic based airflow.

The Jet Stream continues to be progged blowing over the UK over the next 3-4 days. As its pumped up a bit by the new Low early next week the flow splits with the Southern arm disrupting and dissolving away with a new arm taking control with its flow running across the North of Scotland in a week or so time.

UKMO at midday Tuesday shows Low pressure centred over England with rain or showers for all through the day.

ECM continues its theme of deep Low pressure forming to the North of the UK after the demise of the early week Low meaning a continuation of very unsettled weather next week with showers or longer spells of rain with strong winds at times too.

In Summary the weather looks like remaining typically Autumnal this evening. There is nothing out of the ordinary shown within any of the output that one would not be expecting to see in October. So the basic theme of Low pressure dominating close to Northern Britain with bands of rain and wind moving East across the UK periodically with sunshine and showers in between. Temperatures though never warm will not be a million miles from the long term average for mid October. This does not preclude the chance of snow on Scottish mountains or a touch of frost if any ridges pass through overnight.

[phil nw.]

Thanks Gibby, the GFS Ens.Graph underlines the fairly typical Autumn conditions showing in the outputs.

The 850hPa temperatures fairly close to the mean just varying slightly above and below as frontal systems pass through from time to time introducing the change of airmass.

The UKMO image for T144hrs is fairly typical of the modelling for next week showing a broad Westerly flow.

Low pressure crossing the country soon to by followed by a weak ridge.

Edited October 3, 2012 by phil n.warks.

[johnholmes]

Attempting to answer your query re the temperatures.

These are my short hand notes I make each day as I store the anomaly charts-for today

Wed 3 oct

Ec-gfs

Some differences in emphasis re the uk area but essentially a 500 flow from n of west on both, and each having a fairly marked trough with –ve heights; gfs is still a flatter type of flow than ec

Differences over states with ec having little ridging between the eastern and western troughs but gfs showing a quite large +ve area/ridge over nw’ern areas, way theu both treat slight +ve area over atlantic is very similar

To me it looks a mobile rather than a static pattern?

Ec shows heights over far south of uk as around 546 gfs is maybe 4dm or so higher

Noaa with a wly not n of west flow showed much higher 558dm or so in the south

IF noaa follows then its looking a cold not warm spell and unsettled if it does not then unsettled is ok but temperature levels are less clear.

Perhaps you took my comment a touch too literally as what I was trying to get across was the lack of any real ‘warm’ air on the two I showed and the 3 had previously shown a westerly component at 500mb. Neither flow would give ‘warm’ surface temperatures for the majority of the UK. As NOAA this evening suggests with their 8-14 day chart the probability of high pressure close to the SW of the UK does look better than evens, something the two I linked too this morning did not but note my proviso in my notes.

Many years ago we used the 546DM line as the most likely boundary between Polar and Tropical air-a VERY rough initial guide when starting to prepare a media forecast. NOAA is often 5-8DM higher than the other two for a reason I have never been able to fathom out in spite of exchanging e mails with their web site. It does show heights(on the 8-14 day issue) in the Channel area of about 558DM some 10-12DM higher than the other two so my caution this morning is perhaps justified.

The other feature to take into account is the degree of unsettled that any of the charts suggest, whilst another factor that a forecaster using these type of charts must consider is the difference in the NOAA output between the 6-10 and 8-14 day outlooks. One would expect a slight difference but not the substantial differences they quite often show and keep over several days. Another thing I find a bit puzzling. The 6-10 this evening is very much like the ECMWF-GFS issue this morning so I am minded to accept all 3 upper air predictions as they are very similar for the 6-10 day at least.

Overall after all this I would stand by my initial remark this morning with one alteration. ‘Away’ from the SW.

I hope that helps to understand my thinking behind the post I made this morning?

[summer blizzard]

That atlantic high anomaly is quite persistent at the moment.

The anomaly chart for September just gone is fairly similar as well.

Looks like an El Nino pattern over the USA with a +PNA and a southern stream.

[summer blizzard]

Will be interesting to monitor the polar outbreak indicated by the ensembles around the 13th (isotherm around 0C).

[syed2878]

Hi guys noaa daily discussions.

PRELIMINARY EXTENDED FORECAST DISCUSSION.

919 AM EDT WED OCT 03 2012

VALID 12Z SUN OCT 07 2012 - 12Z WED OCT 10 2012

...RELATIVELY COOL AND UNSETTLED CONDITIONS ACROSS MUCH OF THE

LOWER 48...

GENERAL FLOW PATTERN

======================================

THE GUIDANCE IS IN GENERAL AGREEMENT THAT THE REX BLOCK IN THE

EASTERN PACIFIC WILL TRANSFORM INTO A FULL LATITUDE RIDGE NEAR THE

130TH MERIDIAN /OFFSHORE THE WEST COAST OF THE UNITED STATES/ AS A

DEEP CYCLONE MOVES FROM THE SUBTROPICAL PACIFIC INTO THE WEST.

TELECONNECTIONS TO THE RIDGE NEAR SE AK SUGGEST THAT BROAD

TROUGHING SHOULD CENTER NEAR 90W /GREAT LAKES TO MS VALLEY/ WITH

PERIODIC CLIPPER SYSTEMS EXPECTED TO SWEEP DOWN TOWARDS THE

US/CANADIAN BORDER TO REINFORCE THE TROUGH. STANDARDIZED ANOMALIES

OF THE 500MB HEIGHTS ASSOCIATED WITH THE RIDGE/TROUGH COUPLET ARE

ON THE ORDER OF +2.5/-2.5 SO A COOLER THAN AVERAGE FEW DAYS IS IN

STORE FOR ESPECIALLY THE CENTRAL CONUS.

MODEL PREFERENCES

=================

THE DETERMINISTIC AND ENSEMBLE MEAN GUIDANCE WERE IN GOOD

AGREEMENT... INCLUDING MANY SMALLER DETAILS. WHEN COMBINED WITH

THE AMPLIFIED FLOW PATTERN EXPECTED... CONFIDENCE IS ABOVE AVERAGE

IN THE MEDIUM RANGE FORECAST TODAY. THE PRESSURES WERE BASED ON A

40/30/30 BLEND OF THE 00Z UKMET/00Z ECMWF/00Z GFS THROUGH MONDAY

BEFORE SWITCHING TO A 50/50 BLEND OF THE 00Z GFS/00Z ECMWF TUESDAY

AND NEXT WEDNESDAY. THE ECMWF AND GFS ENSEMBLE MEANS SHOWED VERY

GOOD AGREEMENT WITH THEIR OPERATIONAL RUNS AND EACH OTHER THROUGH

ABOUT TUE/D6. THE GUIDANCE DIFFERS FARTHER UPSTREAM OVER ALASKA

WITH THE EVOLUTION OF THE UPPER RIDGE AND WHETHER OR NOT IT MAY

BECOME SO STRONG THAT IT PINCHES OFF AN UPPER HIGH WHICH ALLOWS

ENERGY TO SLIDE THROUGH NW CANADA AND RAISE HEIGHT A BIT THROUGH

THE NORTHERN ROCKIES /ECMWF SOLUTION/. FOR NOW... MANUALLY NUDGED

THE WED/D7 FORECAST TOWARD THE GFS/GEFS MEAN/ECMWF ENSEMBLE MEAN

CLUSTER WHICH KEEPS A STRONG RIDGE INTACT THROUGH THE NE PACIFIC.

THIS FITS WITH THE LAGGED AVERAGE OF THE COMBINED GFS/ECMWF MODELS

OVER THE PAST FOUR 00Z/12Z RUNS. OVERALL THIS KEEPS EXCELLENT

CONTINUITY FROM THE FORECAST 24 HRS AGO.

WEATHER IMPACTS

===============

THE PACIFIC NORTHWEST SHOULD REMAIN HIGH AND DRY UNDER THE AEGIS

OF A WARM CORE RIDGE THROUGH THE PERIOD. DOWNSTREAM... THE

AMPLITUDE OF THE MID-CONTINENTAL TROUGH IS EXPECTED TO KEEP

CONDITIONS COOL OVER MUCH OF THE LOWER 48. THIS COULD RESULT IN

TEMPERATURES AS MUCH AS 20-30F BELOW NORMAL FROM THE

CENTRAL/SOUTHERN PLAINS INTO TEXAS OVER THE WEEKEND BEFORE

MODERATING THEREAFTER. LAKE ENHANCED RAIN/SNOW IS ANTICIPATED FOR

THE GREAT LAKES SAT/SUN AS A SHORTWAVE MOVES ACROSS THE NORTHERN

TIER OF THE COUNTRY. THIS SYSTEM WILL QUICKLY SPREAD PRECIPITATION

EASTWARD FROM THE SOUTHERN PLAINS... WHERE THE RAIN IS SORELY

NEEDED... ACROSS THE MID-MISSISSIPPI AND OHIO VALLEYS INTO THE

MID-ATLANTIC/NEW ENGLAND STATES THIS WEEKEND. AN EARLY-SEASON

CLIPPER SYSTEM MOVING OUT OF WESTERN CANADA COULD BRING A SECOND

CHANCE FOR SNOW TO NORTH DAKOTA AND MINNESOTA BETWEEN SUNDAY NIGHT

AND TUESDAY... WITH COLD RAINS EXPECTED TO SPREAD ACROSS THE GREAT

LAKES MONDAY INTO LATE TUESDAY. MEANWHILE... MODERATE TO LOCALLY

HEAVY RAINS SHOULD PERSIST OVER THE FLORIDA PENINSULA THROUGH AT

LEAST SUNDAY... ADDING TO LOCAL RAINFALL SURPLUSES ACROSS

SOUTHEAST FLORIDA... AS A SUCCESSION OF FRONTAL BOUNDARIES TAP

INTO MOISTURE FROM THE WESTERN CARIBBEAN AND GULF OF MEXICO.

THE CLOSED LOW MOVING INTO CALIFORNIA LATE MONDAY WILL COOL OFF

PORTIONS OF THE WEST AND MOJAVE DESERT AS IT PROGRESSES THROUGH

THE REGION. THE SYSTEM COULD LEAD TO UPSLOPE AND INSTABILITY

PRECIPITATION UNDER THE UPPER-LEVEL COOL POOL ACROSS THE SISKIYOU

AND SIERRA NEVADA RANGES SUNDAY INTO NEXT TUESDAY BEFORE SPREADING

HIGH ELEVATION SNOWS/LOWER ELEVATION RAINS ACROSS THE GREAT

BASIN/CENTRAL ROCKIES/CENTRAL PLAINS TUESDAY INTO NEXT

WEDNESDAY... HELPING TO FURTHER RELIEVE DROUGHT CONDITIONS IN THE

CENTRAL PLAINS. A SECOND CLIPPER SYSTEM WAITS IN THE WINGS ACROSS

WESTERN CANADA TUESDAY INTO NEXT WEDNESDAY... BUT IT APPEARS THAT

IT SHOULD PASS NORTH OF THE CENTRAL US/CANADIAN BORDER LATE NEXT

WEDNESDAY INTO THURSDAY.

ROTH/FRACASSO.

http://www.hpc.ncep.noaa.gov/preepd/preepd.html

[IDO]

Attempting to answer your query re the temperatures.

These are my short hand notes I make each day as I store the anomaly charts-for today

Wed 3 oct

Ec-gfs

Some differences in emphasis re the uk area but essentially a 500 flow from n of west on both, and each having a fairly marked trough with –ve heights; gfs is still a flatter type of flow than ec

Differences over states with ec having little ridging between the eastern and western troughs but gfs showing a quite large +ve area/ridge over nw’ern areas, way theu both treat slight +ve area over atlantic is very similar

To me it looks a mobile rather than a static pattern?

Ec shows heights over far south of uk as around 546 gfs is maybe 4dm or so higher

Noaa with a wly not n of west flow showed much higher 558dm or so in the south

IF noaa follows then its looking a cold not warm spell and unsettled if it does not then unsettled is ok but temperature levels are less clear.

Perhaps you took my comment a touch too literally as what I was trying to get across was the lack of any real ‘warm’ air on the two I showed and the 3 had previously shown a westerly component at 500mb. Neither flow would give ‘warm’ surface temperatures for the majority of the UK. As NOAA this evening suggests with their 8-14 day chart the probability of high pressure close to the SW of the UK does look better than evens, something the two I linked too this morning did not but note my proviso in my notes.

Many years ago we used the 546DM line as the most likely boundary between Polar and Tropical air-a VERY rough initial guide when starting to prepare a media forecast. NOAA is often 5-8DM higher than the other two for a reason I have never been able to fathom out in spite of exchanging e mails with their web site. It does show heights(on the 8-14 day issue) in the Channel area of about 558DM some 10-12DM higher than the other two so my caution this morning is perhaps justified.

The other feature to take into account is the degree of unsettled that any of the charts suggest, whilst another factor that a forecaster using these type of charts must consider is the difference in the NOAA output between the 6-10 and 8-14 day outlooks. One would expect a slight difference but not the substantial differences they quite often show and keep over several days. Another thing I find a bit puzzling. The 6-10 this evening is very much like the ECMWF-GFS issue this morning so I am minded to accept all 3 upper air predictions as they are very similar for the 6-10 day at least.

Overall after all this I would stand by my initial remark this morning with one alteration. ‘Away’ from the SW.

I hope that helps to understand my thinking behind the post I made this morning?

Very helpful. It is such a steep learning curve for us newbies and explanations like this assist in helping understand the pro-posters. I get the emphasis on the post now. Thanks.

[IDO]

The GFS 0z is a bit of a dog's dinner next week, with lows and highs mixing it up.

After T240 (FI), this run has a Scandi High in a blocked pattern with an Atlantic high, for 7 days. The UK seemingly right in the middle and with the split dominant part of the Jetstream visiting North Africa, one to watch.

But its a slack pattern next week, on this run, so average temps and drizzly, look the main synoptics. However I get the impression GFS is still not sure past T120; with HP and LP battling it out.

[Frosty.]

There was major differences between the gfs00z and ecm12z ens mean this morning although the ecm mean (00z) has flipped towards the gfs 00z, but as things stood, the ecm mean from last night showed a cool and unsettled spell developing next week and persisting through to T+240 hours and well beyond with a polar maritime flow which would bring wintry showers to northern hills and especially the scottish mountains. The gfs 00z mean is much different and shows an anticyclonic spell for most of the uk for a big chunk of next week, especially for the southern half of the uk but then a brief cold shot from the nw by day 9 (T+216 Hours) before atlantic hp moves into the uk, the further outlook then shows high pressure migrating further to the southwest with a cooler and unsettled flow developing from north of west, similar to the ecm 12z mean by the end.

Edit. Ecm 00z mean has changed for the better as Gibby said.

Edited October 4, 2012 by Frosty039

[Gibby]

Good morning. Let's see what can be made of the 00z output this morning.

All models show a frontal wave crossing Southern Britain later tonight and straddling itself across Southern Britain through Friday while the North maintains itself in a weakening and showery westerly flow still. Through Saturday a ridge of High pressure transfers across the UK though UKMO worryingly keeps Fridays trough perilously close to the south with the chance of cloudy conditions maintained in the South should it verify. On Sunday Low pressure trundles NE towards Britain carrying rain and freshening winds NE across Britain over Sunday.

GFS then takes us into next week with the Low pressure responsible for sundays rain moving slowly NE across Britain filling as it goes with a ridge approaching Scotland from the NW. Rain or showers would occur almost everywhere away from the far NW where it would dry up later. The ridge then drifts slowly across Scotland and into the North Sea though weak troughs will be caught up in it's circulation keeping a lot of cloud going with some light rain around chiefly in the south. as we move through FI Low pressure is shown to move SE from Iceland and down the North Sea with rain at times and rather cold weather in a Northerly breeze . In the final third of the run the weather turns very unsettled over Southern Britain with Low pressure over Europe and High over Scandinavia with the driest conditions in the North. It would turn rather chilly everywhere if this set up verified.

The GFS Ensembles show a distinct drop in temperatures for Southern locations in the second half of the run with some members going quite cold for this stage in October. The operational run was one of the milder options in a very changeable pattern. The North shows a somewhat less stark fall with near normal or just dlightly below uppers more likely.

The Jet Stream shows a continuation of the flow across the UK for three to four more days. The flow ridges through the Atlantic and splits by early next week with the Southern arm quickly weakening as the Northern arm blows then towards Scotland later next week.

UKMO for midnight on shows Low pressure over Scotland with a trough over the UK. Rain or showers would be crossing East through the day in brisk SW winds veering NW later in the day.

ECM is much better this morning and although the trend is still for an unsettled spell Southern areas will get away much lighter on this run with less rain and drier periods in between. The North though will continue to see wind and rain feature prominently.

In Summary the weather looks like staying changeable through the period. There is still much confusion between the models on how things are likely to evolve next week with ECM bringing less cool and drier conditions for the South as the Azores High inches close while GFS brings a cool set of ensembles later and low pressure to the South while High pressure builds over Scandinavia. There is still much to play for beyond the weekend over the next few days between the models.

[Gibby]

Will be interesting to see the ECM Ensembles later to see where the operational stands.

Edited October 4, 2012 by Gibby

[bluearmy]

interesting to see naefs has highish height anomolys through next week whilst the models continue to show a depression moving northeast across the uk. maybe it wont be as poor as it currently looks although i cant see it being as decent as those height anomolys currently show.