Update to Tuesday-Wednesday severe weather

This is an update to my post Sunday. I just checked the SPC outlooks, and now it may not be an extreme event after all, perhaps just the usual spring season stuff in the Southeast (which is still not at all fun).

Han

They didn’t go to a moderate risk after all, and they still aren’t using the “O” word – “outbreak.”

Yay! But don’t get cocky, kid – keep a weather eye out, and listen to the radio and TV.

Of course you also have a weather radio and batteries, right?

Tuesday

I’ve highlighted relevant parts of today’s day 2 discussion below. (ATTM means “at this time.”)

day2probotlk_0600_any

SPC AC 080553

DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1253 AM CDT MON APR 08 2013

VALID 091200Z – 101200Z

…THERE IS A SLGT RISK OF SVR TSTMS FROM THE LOWER MO VALLEY TO THE
SRN PLAINS…

…SYNOPSIS…
HIGHLY AMPLIFIED UPPER-LEVEL TROUGH OVER THE WEST SHOULD EVOLVE INTO
A FEW DISTINCT SHORTWAVE IMPULSES…TWO OF WHICH WILL BE PROMINENT
FOR AFFECTING SEVERE POTENTIAL IN THE CNTRL STATES. ONE IMPULSE
SHOULD MOVE NEWD TOWARDS THE CNTRL HIGH PLAINS/BLACK HILLS…WHILE
ANOTHER SHOULD EJECT ONTO THE SRN HIGH PLAINS TUE NIGHT. AT THE
SURFACE…A DRYLINE WILL MIX EWD INTO WRN OK AND ACROSS THE EDWARDS
PLATEAU IN TX. A SHARP COLD FRONT WILL PROGRESSIVELY OVERTAKE THE
DRYLINE…LIKELY REACHING A NERN TO S-CNTRL TX LINE BY 12Z/WED.

…LOWER MO VALLEY TO THE SRN PLAINS…
A BROAD CORRIDOR OF SEVERE TSTMS WITH ALL HAZARD TYPES POSSIBLE IS
EXPECTED TO COMMENCE TUE AFTERNOON AND CONTINUE THROUGH WED MORNING.

NCEP GUIDANCE HAS FINALLY COME INTO LINE WITH THE MORE CONSISTENT
ECMWF WITH DETAILS OVER THE SYNOPTIC PATTERN…RENDERING HIGHER
CONFIDENCE IN THE ANTICIPATED CONVECTIVE EVOLUTION. NEVERTHELESS… A
FEW LIMITING FACTORS HAVE MITIGATED A MODERATE RISK ISSUANCE ATTM. A
MODERATELY UNSTABLE AIR MASS WILL BECOME MORE WIDESPREAD WITH LOWER
TO MIDDLE 60S SURFACE DEW POINTS IN MOST OF THE S-CNTRL
CONUS…BENEATH A STOUT CAPPING INVERSION THAT SHOULD LARGELY
INHIBIT WARM SECTOR/DRYLINE TSTM DEVELOPMENT. SHORTWAVE IMPULSES
EJECTING FROM THE WRN CONUS TROUGH ALSO DO NOT APPEAR FAVORABLY
TIMED FOR THE DIURNAL HEATING CYCLE…WITH GREATER MID-LEVEL HEIGHT
FALLS NOT ANTICIPATED TO OVERSPREAD THE WARM SECTOR UNTIL THE LATTER
HALF OF THE PERIOD. DAYTIME TSTM INITIATION WILL LIKELY BE TIED TO
THE COLD FRONT AND NEAR THE INTERSECTION OF THE MERGING COLD
FRONT/DRYLINE…WITH TSTM COVERAGE SUBSTANTIALLY INCREASING DURING
THE EVENING/OVERNIGHT. THE UNDERCUTTING NATURE OF THE COLD FRONT
SHOULD LIMIT THE SUSTAINABILITY OF ROBUST INDIVIDUAL UPDRAFTS.

NEVERTHELESS…STEEP MID-LEVEL LAPSE RATES AND STRONG DEEP-LAYER
SHEAR WILL BE FAVORABLE FOR SUPERCELL CLUSTERS INITIALLY PRODUCING
LARGE HAIL /SOME SIGNIFICANT/ ALONG WITH A FEW TORNADOES AND
ISOLATED DAMAGING WINDS.
WITH TIME…AN EXTENSIVE LINE OF CONVECTION
SHOULD FORM BUT AS THE COLD FRONT BECOMES INCREASINGLY
ANAFRONTAL…SEVERE HAIL/WIND RISKS MAY BE SUBDUED. STILL…SOME
INCREASE IN TORNADO AND DAMAGING WIND THREAT IS POSSIBLE EARLY WED
MAINLY IN NERN TX/WRN AR AS LOW-LEVEL SLYS STRENGTHEN WITH APPROACH
OF THE SRN HIGH PLAINS IMPULSE.

..GRAMS.. 04/08/2013

Storms need convection to form and grow. You’ve seen cauliflower-shaped clouds soaring upward in summer skies, perhaps – that’s convection in action.

Meteorologists sometimes call severe weather “convective weather” – that’s how fundamental convection is to the whole process.

The SPC forecaster mentions a cap in their day 2 outlook. In plain English, this means the upper levels of the atmosphere aren’t going to favor storm growth – they’ll be capped.

Severe weather can and probably will happen, but now there’s a limit to just how much convection can get going.

Yay!

If you didn’t tune out that technical stuff and really want to know more details about weather, though perhaps not to the degree required to become a meteorologist, perhaps MetEd would interest you. Registration is required, but it’s free and they don’t spam you. There is a whole range of information levels available, and much of this is in plain English and easily followed by anyone with a basic science background.

The forecaster also noted other mitigating factors, including timing.

If you’ve been through severe weather before, you know that it’s worst when the sun is out and sometimes peters out once the sun goes down. Apparently that’s going to be the case with this system on Tuesday. Again, yay!

Wednesday

It looks like “just” another severe weather day for the rather unfortunately outlined risk areas.

day3prob_0730

DAY 3 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0228 AM CDT MON APR 08 2013

VALID 101200Z – 111200Z

…THERE IS A SLGT RISK OF SVR TSTMS FROM PARTS OF THE MIDWEST TO
THE WRN GULF COAST STATES…

…SYNOPSIS…
AN AMPLIFIED UPPER-LEVEL TROUGH WILL CONTINUE TO GRADUALLY PROGRESS
EWD FROM THE ROCKIES ACROSS THE GREAT PLAINS THROUGH EARLY THU. A
DOWNSTREAM MID-LEVEL RIDGE WILL PERSIST FROM THE SOUTHEAST TO THE
GREAT LAKES. AT THE SURFACE…A CYCLONE SHOULD SLOWLY MOVE EWD IN
PARTS OF THE MIDWEST WITH AN ATTENDANT COLD FRONT FROM THE LOWER MO
VALLEY TO THE SRN PLAINS PROGRESSING AT LEAST ACROSS THE MID/LOWER
MS VALLEY BY 12Z/THU. A QUASI-STATIONARY FRONT SHOULD BE DRAPED E OF
THE MIDWEST CYCLONE TO THE NRN APPALACHIANS.

…MIDWEST TO THE WRN GULF COAST STATES…
A BROAD SWATH OF ORGANIZED SEVERE POTENTIAL IS EXPECTED DURING THE
PERIOD WITH ALL SEVERE HAZARDS PROBABLE.

MODEL PREDICTABILITY LESSENS SUBSTANTIALLY FROM D2 WITH THE
EVOLUTION OF KEY SYNOPTIC FEATURES…RENDERING LOW CONFIDENCE IN
BOTH THE AMPLITUDE OF THE SEVERE THREAT AND IN IDENTIFYING ANY
CORRIDOR OF COVERAGE AOA 45 PERCENT. MODELS DIFFER WITH THE EJECTION
OF A SRN HIGH PLAINS SHORTWAVE IMPULSE…WITH THE MORE CONSISTENT
ECMWF/CMC SUGGESTING A SLOWER AND MORE N/NELY TRACK TOWARDS THE
MID-MO VALLEY. THIS YIELDS A SLOWER EWD PROGRESSION OF THE SURFACE
COLD FRONT AND LOW/MID-LEVEL S/SWLYS NOT AS INTENSE OVER THE WARM
SECTOR COMPARED TO THE GFS/NAM.

REGARDLESS OF THE MODEL DISCREPANCIES…SCATTERED TO WIDESPREAD
CONVECTION IS EXPECTED ALONG THE FRONTAL ZONES. SURFACE DEW POINTS
WILL CONTINUE TO INCREASE WITHIN THE WARM SECTOR…LIKELY THROUGH
THE 60S FROM PARTS OF THE MIDWEST TO THE GULF COAST. WITH A REMNANT
EML PLUME OVER MOST OF THE ERN CONUS CONTRIBUTING TO STEEP MID-LEVEL
LAPSE RATES…STRENGTHENING LOW/DEEP-LAYER SHEAR SHOULD PROVE
FAVORABLE FOR SCATTERED DAMAGING WINDS…A FEW TORNADOES…AND
ISOLATED LARGE HAIL. AS FLOW BECOMES INCREASINGLY PARALLEL TO THE
COLD FRONT…AN EXTENSIVE QLCS MAY ULTIMATELY FORM PRIOR TO THE
FRONT REACHING THE MS VALLEY.

..GRAMS.. 04/08/2013

You have probably seen a quasilinear convective system (QLCS) on TV during a local severe weather event:

Source, with information.

Radar image of a QLCS in 2009. Source, with information.)

It’s a squall line.

It looks like this system on Wednesday is now expected to angle up more towards the northeast and may not impact most of the Southeast. Remember also what they said at the end of the day 2 outlook: “some increase in tornado and damaging wind threat is possible early Wed mainly in NERN TX/WRN (Arkansas).”

The rest of the week

The SPC’s day 4-8 outlook is blank, with the note “predictability too low.” This doesn’t mean there won’t be thunderstorms and/or severe weather, only that they are waiting to see how things develop, especially on Tuesday.

A question here is just how large the scale of Wednesday’s QLCS may be. It’s early in the year for a derecho, but that’s something to keep at the back of one’s mind.

Stay safe out there, people! Thanks for your interest.



Categories: Weather

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