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April 21-24th Severe Weather


SEMIweather

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04/21: the GFS indicates a fairly volatile, but capped, environment over western portions of the Southern Plains as a low-amplitude trough ejects from the Rockies. The H7 temperatures seem marginal, at best, for the prospects of surface-based convection in April. One positive is that moisture return by Thursday morning looks relatively plentiful on both the GFS and NAM. Additionally, directional shear should be quite good. Unfortunately, the ECMWF this morning looked even less amplified, which would be problematic. Basically, this looks like a highly-conditional severe threat. Very low-amplitude troughs tend to be that way regardless, but with the particularly warm EML's we're seeing this spring, initiation may be even more difficult IMO.

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04/21: the GFS indicates a fairly volatile, but capped, environment over western portions of the Southern Plains as a low-amplitude trough ejects from the Rockies. The H7 temperatures seem marginal, at best, for the prospects of surface-based convection in April. One positive is that moisture return by Thursday morning looks relatively plentiful on both the GFS and NAM. Additionally, directional shear should be quite good. Unfortunately, the ECMWF this morning looked even less amplified, which would be problematic. Basically, this looks like a highly-conditional severe threat. Very low-amplitude troughs tend to be that way regardless, but with the particularly warm EML's we're seeing this spring, initiation may be even more difficult IMO.

Hasn't the Euro been providing flatter, less amplified solutions lately, only to transition a sharper, more GFS-like solution in later runs? Maybe, it's me, but it seems that way.

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Hasn't the Euro been providing flatter, less amplified solutions lately, only to transition a sharper, more GFS-like solution in later runs? Maybe, it's me, but it seems that way.

It has, and it's been wrong lately at this range. It also is the furthest SE on both systems this week.

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Tomorrow's system may slow down into a pseudo 50/50 low as it departs off New England, locking southerly flow over the plains. I'm liking the fact that an upper level disturbance off the mexican coast south of baja can be seen in the GFS and the NAM which would mean low pressure stretching from Mexico to the primary in the front range. Shear may not be as impressive as some of the events we've seen recently, but moisture return looks improved west of I-35, although east of 35 is still the much better bet. Hopefully the euro comes into better agreement tonight.

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Tomorrow's system may slow down into a pseudo 50/50 low as it departs off New England, locking southerly flow over the plains. I'm liking the fact that an upper level disturbance off the mexican coast south of baja can be seen in the GFS and the NAM which would mean low pressure stretching from Mexico to the primary in the front range. Shear may not be as impressive as some of the events we've seen recently, but moisture return looks improved west of I-35, although east of 35 is still the much better bet. Hopefully the euro comes into better agreement tonight.

For Thursday, the new NAM says:

Really getting the feeling that this spring will be a tough pill for most Plains chasers to swallow, perhaps moreso than 2009 and comparable to 2006.

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With the NAM & GFS both consistently showing good instability, decent shear, and convective initiation on Friday...I would be surprised if the SPC did not put a Slight Risk out over portions of Oklahoma in the upcoming Day 3 Outlook.

There's still a ton of disagreement between the NAM & GFS on how things will unfold Thursday; GFS shows the instability getting much further north than what the NAM shows.

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New Day 3

day3prob_0730.gif

DAY 3 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

0228 AM CDT WED APR 20 2011

VALID 221200Z - 231200Z

...THERE IS A SLGT RISK OF SVR TSTMS FROM THE SOUTHERN PLAINS TO THE

MIDDLE MS/LOWER OH RIVER VALLEYS...

...SOUTHERN PLAINS/LOWER MO VALLEY TO THE MIDDLE MS/LOWER OH RIVER

VALLEYS...

AT LEAST MODEST HEIGHT FALLS WILL CONTINUE TO OVERSPREAD MUCH OF THE

CENTRAL STATES ON FRIDAY AS A SHORTWAVE TROUGH/ATTENDANT POLAR JET

ARE EXPECTED TO ADVANCE NORTHEASTWARD OVER THE UPPER MIDWEST TOWARD

THE UPPER MS VALLEY/UPPER GREAT LAKES REGION. A CORRESPONDING

SURFACE LOW SEEMS LIKELY TO ADVANCE/POTENTIALLY DEEPEN NORTHEASTWARD

FROM KS EARLY FRIDAY TO THE UPPER MS RIVER VALLEY/UPPER GREAT LAKES

VICINITY FRIDAY NIGHT...WHILE A SECONDARY SURFACE LOW REMAINS

ANCHORED ACROSS NORTHWEST TX/FAR WESTERN OK.

INITIALLY...AT THE TERMINUS OF A STRONG/VEERING LOW LEVEL

JET...ELEVATED TSTMS CAPABLE OF SEVERE HAIL ARE LIKELY TO BE ONGOING

FRIDAY MORNING ACROSS THE LOWER MO VALLEY/OZARKS TOWARD THE LOWER OH

RIVER VALLEY. WHILE THE EXACT IMPACTS OF THIS EARLY DAY CONVECTION

ARE UNCERTAIN...AS THIS ACTIVITY COULD EFFECTIVELY IMPEDE THE DEGREE

OF MOISTENING/DESTABILIZATION WITH NORTHWARD EXTENT...AN

INCREASINGLY MOIST AIRMASS /LOWER TO MIDDLE 60S F SURFACE DEWPOINTS/

WILL OTHERWISE CONTINUE TO RETURN NORTH-NORTHEASTWARD ACROSS THE

MIDDLE MS/LOWER OH VALLEYS AHEAD OF AN ADVANCING SURFACE LOW AND

NORTHEAST-SOUTHWEST ORIENTED COLD FRONT.

AS THE PRE-COLD FRONTAL AIRMASS BECOMES MODERATELY UNSTABLE DURING

THE AFTERNOON...CURRENT THINKING IS THAT SURFACE BASED TSTMS WILL

DEVELOP BY LATE AFTERNOON/EARLY EVENING...ESPECIALLY ACROSS

MO/PERHAPS WESTERN IL ALONG/AHEAD OF THE COLD FRONT...SOUTHWESTWARD

INTO OK NEAR/EAST OF AN EXPECTED SURFACE LOW AND FRONTAL

BOUNDARY/DRYLINE TRIPLE POINT ACROSS WESTERN OK. GIVEN MODERATELY

STRONG CYCLONIC UPPER FLOW ALOFT /40-55 KT AROUND 500 MB/ COLOCATED

WITH THE FRONTAL ZONE/ADJACENT MOIST AXIS...INITIAL MODAL SUPERCELLS

AND SUBSEQUENTLY EVOLVING LINEAR CLUSTERS /DURING THE EVENING/ WILL

POSE AN INITIAL LARGE HAIL THREAT IN ADDITION TO THE POSSIBILITY OF

TORNADOES AND DAMAGING WINDS. THE SEVERE THREAT WILL LIKELY DEVELOP

EASTWARD ACROSS THE OH VALLEY FRIDAY NIGHT.

FARTHER SOUTH ACROSS TX...OTHER MORE ISOLATED SEVERE TSTMS /HAIL AS

THE MAIN THREAT/ MAY DEVELOP IN VICINITY OF THE DRYLINE ACROSS

WEST-CENTRAL TX FRIDAY AFTERNOON/EARLY EVENING.

..GUYER.. 04/20/2011

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PAH

AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE PADUCAH KY 420 PM CDT WED APR 20 2011 .DISCUSSION... RUNNING WAY BEHIND DUE TO MAJOR MOP UP DUTY AFTER LAST NIGHTS WIDESPREAD SEVERE WIND EVENT. NO HUGE CHANGES...AS WE ARE DEFINITELY IN A VERY WET AND POTENTIALLY DANGEROUS PATTERN.

Hope alls well with your home Beau. Very active indeed. reminds me of 03 pattern for activities sake. We experienced alot of severe weather yesterday.

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I have been watching this one for several days now and to me it looks like a classic IL warm front tornado event Friday afternoon through evening. There appears to be more shear than yesterday's event except with a little less instability perhaps do to morning precip. I was not able to go out chasing yesterday, however I will be able to on Friday. It's been almost 1.5 years since I have seen a tornado because I have been way too busy with my 16 month old daughter and was not able to go out to the Plains at all last year. This year I will be on a mini trip May 26-May 30 which means there will be a death ridge during that timeframe. "If" the 18z models verify this may be the best shot in a while of myself seeing long lasting sups and probable tornadoes near the warm front Friday, :thumbsup:

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I have been watching this one for several days now and to me it looks like a classic IL warm front tornado event Friday afternoon through evening. There appears to be more shear than yesterday's event except with a little less instability perhaps do to morning precip. I was not able to go out chasing yesterday, however I will be able to on Friday. It's been almost 1.5 years since I have seen a tornado because I have been way too busy with my 16 month old daughter and was not able to go out to the Plains at all last year. This year I will be on a mini trip May 26-May 30 which means there will be a death ridge during that timeframe. "If" the 18z models verify this may be the best shot in a while of myself seeing long lasting sups and probable tornadoes near the warm front Friday, :thumbsup:

Hope the traditional Tazewell county hotspot performs well for you!

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day2probotlk_0600_any.gif

DAY 2 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

1258 AM CDT THU APR 21 2011

VALID 221200Z - 231200Z

...THERE IS A SLGT RISK OF SVR TSTMS FROM THE SOUTHERN PLAINS/OZARKS

TO THE MIDDLE MS/OH RIVER VALLEYS...

...SYNOPSIS...

A SLIGHT AMPLIFICATION OF THE LARGE SCALE PATTERN WILL CONTINUE TO

OCCUR INTO FRIDAY...AS AT LEAST MODEST HEIGHT FALLS OVERSPREAD MUCH

OF THE CENTRAL STATES ON FRIDAY AS A SHORTWAVE TROUGH/ATTENDANT

POLAR JET SPREAD NORTHEASTWARD OVER THE UPPER MIDWEST TOWARD THE

UPPER MS VALLEY/UPPER GREAT LAKES REGION. A CORRESPONDING SURFACE

LOW SEEMS LIKELY TO ADVANCE/EVENTUALLY DEEPEN NORTHEASTWARD FROM THE

SOUTH-CENTRAL PLAINS EARLY FRIDAY TO THE UPPER MS RIVER VALLEY/UPPER

GREAT LAKES VICINITY FRIDAY NIGHT...WHILE A SECONDARY SURFACE LOW

WILL BE ANCHORED ON THE WEST EDGE OF THE MOIST SECTOR ACROSS

NORTHWEST TX/FAR SOUTHWEST OK.

...SOUTHERN PLAINS/OZARKS TO THE MIDDLE MS/LOWER OH VALLEYS...

INITIALLY...AT THE TERMINUS OF A STRONG/SPLIT AND GRADUALLY VEERING

LOW LEVEL JET...ELEVATED TSTMS CAPABLE OF SEVERE HAIL ARE LIKELY TO

BE ONGOING FRIDAY MORNING WITHIN A CORRIDOR ACROSS THE LOWER MO

VALLEY/MIDDLE MS VALLEY TOWARD THE LOWER OH VALLEY. AN ISOLATED

SEVERE HAIL THREAT MAY CONTINUE THROUGH THE MORNING AS STORMS SPREAD

EASTWARD ON THE EDGE OF THE EASTWARD ADVANCING LOWER MO VALLEY

ELEVATED MIXED LAYER.

THIS EARLY DAY CONVECTION CREATES A DEGREE OF UNCERTAINTY REGARDING

THE SPATIAL EXTENT OF THE SUBSEQUENT AFTERNOON/EVENING WARM/MOIST

SECTOR FOR AREAS SUCH AS NORTHEAST MO INTO CENTRAL/SOUTHERN PORTIONS

OF IL/INDIANA...AS THIS EARLY DAY ACTIVITY COULD EFFECTIVELY IMPEDE

THE DEGREE OF MOISTENING/SURFACE BASED DESTABILIZATION WITH

NORTHWARD EXTENT. REGARDLESS...AN INCREASINGLY MOIST

AIRMASS...CHARACTERIZED BY LOWER TO MIDDLE 60S F SURFACE DEWPOINTS

WILL OTHERWISE QUICKLY RETURN NORTH-NORTHEASTWARD ACROSS THE MIDDLE

MS/LOWER OH VALLEYS AHEAD OF THE NORTHEASTWARD ADVANCING MIDWEST

SURFACE LOW AND NORTHEAST-SOUTHWEST ORIENTED COLD FRONT.

THE PRE-COLD FRONTAL AIRMASS WILL LIKELY BECOME MODERATELY TO

STRONGLY UNSTABLE DURING THE AFTERNOON...WITH 1000-2500 J/KG MLCAPE

PROBABLE ACROSS PORTIONS OF MO/DOWNSTATE IL...AND STRONGER 3000+

J/KG MLCAPE ACROSS OK/FAR NORTH TX. CURRENT THINKING IS THAT SURFACE

BASED TSTMS WILL DEVELOP BY LATE AFTERNOON/EARLY

EVENING...ESPECIALLY ACROSS THE EASTERN/SOUTHERN HALVES OF MO INTO

WESTERN IL ALONG/AHEAD OF THE COLD FRONT...AS WELL AS SOUTHWESTWARD

INTO FAR SOUTHEAST KS/FAR NORTHWEST AR...AND OK NEAR/NORTHEAST OF AN

EXPECTED SURFACE LOW AND FRONTAL BOUNDARY/DRYLINE TRIPLE POINT.

GIVEN STRONG CYCLONIC MID-UPPER FLOW ALOFT /45-60 KT AROUND 500 MB/

COLOCATED WITH THE FRONTAL ZONE/ADJACENT MOIST SECTOR...INITIAL

MODAL SUPERCELLS ARE PROBABLE...WITH LARGE HAIL AND TORNADOES

POSSIBLE. CURRENT THINKING IS THE RELATIVELY GREATEST TORNADO THREAT

MAY EXIST ACROSS EAST/SOUTHEAST MO INTO DOWNSTATE IL FRIDAY LATE

AFTERNOON/EARLY EVENING...AHEAD OF THE ADVANCING SURFACE LOW AND

ALONG/JUST SOUTH OF WHAT MAY BE A CONVECTIVELY REINFORCED WEST-EAST

EFFECTIVE FRONTAL ZONE...ALTHOUGH A TORNADO THREAT WILL EXIST

SOUTHWESTWARD INTO OK AS WELL.

BY FRIDAY EVENING...A MORE LINEAR CONVECTIVE MODE IS LIKELY TO

EVOLVE ESPECIALLY ACROSS THE OH VALLEY...WITH UPSCALE GROWING

QUASI-LINEAR CLUSTERS/BOWING SEGMENTS POSING A DAMAGING

WIND/ISOLATED TORNADO RISK FRIDAY NIGHT. FARTHER SOUTHWEST ACROSS

THE REMAINDER OF THE NORTHEAST-SOUTHWEST FRONTAL ZONE...OTHER TSTMS

WILL CONTINUE TO POSE AT LEAST AN ISOLATED SEVERE HAIL THREAT FRIDAY

NIGHT ACROSS SOUTHERN MO/NORTHERN AR INTO OK AMID A WARM

ADVECTION/ISENTROPIC LIFT REGIME ALONG/NORTH OF THE SURFACE FRONT.

...NORTH-CENTRAL/WEST-CENTRAL TX...

OTHER MORE ISOLATED SEVERE TSTMS MAY DEVELOP IN VICINITY OF THE

NORTHEAST-SOUTHWEST ORIENTED DRYLINE FRIDAY AFTERNOON/EARLY EVENING.

WHILE BACKGROUND LARGE SCALE FORCING FOR ASCENT WILL BE NEBULOUS AND

MID TROPOSPHERIC WINDS WILL BE WEAKER WITH SOUTHWARD

EXTENT...VEERING WIND PROFILES/RESERVOIR OF STRONG INSTABILITY WOULD

SUPPORT SOME SUPERCELLS CAPABLE OF LARGE HAIL WITH ANY STORMS THAT

DO FORM.

..GUYER.. 04/21/2011

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I like the new SPC graphics, as they shaded in the regions, but I dont like how they changed the colors for the outlooks.

Great point, now that's going to annoy the living hell out of me. I struggle with change. :P

I guess I can talk myself into this making more sense though. Green, yellow, red (then pink) progression for elevating thunderstorm risk like a traffic light's go, yield, stop.

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Looking at the day before the day everyone is focused on (tornado threat is non-zero after all)...

There will be a nice theta-e boundary just south of the Red River, with storm motions favoring riding along rather than crossing the boundary. However, shear is marginal for supercells and low levels winds are light, so the storms will likely have to generate some of their own vorticity in the low levels. Mid level dry air should support some nice downdrafts, so outflow enhanced boundaries are likely and brief spin ups would not surprise me.

That being said, hail looks to be the greatest threat in the area. The 00z GFS is forecasting over 1000 J/kg CAPE in the hail growth zone (-10 to -30 C). High freezing levels will require some big updrafts to produce severe hail, but given the instability forecast that should be doable. Looking at 50dBZ heights about 36kft south of the Red River, dropping to around 30kft by OKC. You can see the juxtaposition valid at 00z this evening.

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