7/27 Severe Threat Discussion and Obs

[CAPE]

Interesting excerpt from Mt Holly AFD-

 

AS FAR AS MODEL SHORT WAVE VERIFICATION BASED ON 24HR FCSTS ITS THE
ECMWF, THE GFS @ 18HRS, THEN CAN GGEM AND WRF-NMMB. THIS IS BOTH A
VOLATILE AND ANOMALOUS REGIME FOR LATE JULY AND WE ARE GOING TO RUN
WITH THE ASSUMPTION UNTIL THE TROF CLEARS THAT SHORT WAVES WILL
OUTPERFORM. 12Z UA ANALYSIS SHOWED A POCKET OF 19C DEW POINTS AT
850MB/925MB WEST OF CHICAGO. THIS CANT BE DENIED OVERNIGHT AND
CONVECTION IS FIRING OVER NORTHERN INDIANA.

 

Some points well made...this is an anomalous set up for July. There are multiple shortwaves involved and timing and strength will play a role. There was a pretty robust line of storms that developed and moved through here  this morning(not in the forecast as of last night) with a modeled "weak" shortwave that wasn't really supposed to do much of anything. Ended up producing a few warnings SE of here, although generally the storms weren't severe. Maybe this event will be an over-performer

[Kmlwx]

Pattern recognition still says it's worth an extra eye but the GFS has a huge cape hole over us thru much of the day. I dunno.

The question is whether that will actually happen or not. If we clear out at all I doubt that hole will be as obvious. Jet streak being slow has potential to mess things up as you said. 

[Shane M.]

Interesting excerpt from Mt Holly AFD-

 

AS FAR AS MODEL SHORT WAVE VERIFICATION BASED ON 24HR FCSTS ITS THE

ECMWF, THE GFS @ 18HRS, THEN CAN GGEM AND WRF-NMMB. THIS IS BOTH A

VOLATILE AND ANOMALOUS REGIME FOR LATE JULY AND WE ARE GOING TO RUN

WITH THE ASSUMPTION UNTIL THE TROF CLEARS THAT SHORT WAVES WILL

OUTPERFORM. 12Z UA ANALYSIS SHOWED A POCKET OF 19C DEW POINTS AT

850MB/925MB WEST OF CHICAGO. THIS CANT BE DENIED OVERNIGHT AND

CONVECTION IS FIRING OVER NORTHERN INDIANA.

 

Some points well made...this is an anomalous set up for July. There are multiple shortwaves involved and timing and strength will play a role. There was a pretty robust line of storms that developed and moved through here  this morning(not in the forecast as of last night) with a modeled "weak" shortwave that wasn't really supposed to do much of anything. Ended up producing a few warnings SE of here, although generally the storms weren't severe. Maybe this event will be an over-performer

 

You are exactly correct about the storms this morning. I think that is causing Mt. Holly to be a little extra paranoid, which I think is a good thing because they definitely missed the forecast this morning.

 

Here is an excerpt from the earlier AFD

 

VIGOROUS SHORT WAVE HAS COME ACROSS OUR CWA MUCH STRONGER, EARLIER

AND FARTHER NORTH THAN WE WERE EXPECTING YESTERDAY, REASON FOR

THIS MORNING`S SHOWERS AND ISOLATED THUNDERSTORMS.

[olafminesaw]

Would overnight storms help or hinder our severe threat? There appears to be a line of downpours headed our way now, although it will probably weaken as it crosses the mountains.

[Kmlwx]

Would overnight storms help or hinder our severe threat? There appears to be a line of downpours headed our way now, although it will probably weaken as it crosses the mountains.

Overnight storms shouldn't have much of an impact on instability tomorrow unless they stick around into the AM. They could lay down boundaries around the area, though. 

[EasternUSWX]

Would overnight storms help or hinder our severe threat? There appears to be a line of downpours headed our way now, although it will probably weaken as it crosses the mountains.

 

Supposed to help lay down outflow boundaries. As long as the clouds break up by morning we will be fine. 

[mattie g]

Sorry for the simplistic question, but what does it mean when folks say that storms before a main event help to "lay down boundaries?"

[EasternUSWX]

Sorry for the simplistic question, but what does it mean when folks say that storms before a main event help to "lay down boundaries?"

 

Weakening storms leave the outflow boundaries and new thunderstorms can form along these boundaries. 

[snowfan]

The storms that move through tonight, if there are any, could leave outflow boundaries that hang around through tomorrow. Knowing where these are could help figure out where storms fire tomorrow as the boundary can act as a trigger for convection. 

Sorry for the simplistic question, but what does it mean when folks say that storms before a main event help to "lay down boundaries?"

[mattie g]

Thanks for the answers. I didn't realize that outflow boundaries could stick around like that.

[yoda]

Well-written and nice disco update from LWX

 

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
914 PM EDT SAT JUL 26 2014

.SYNOPSIS...
A WARM FRONT WILL LIFT NORTH OF THE AREA EARLY SUNDAY. LOW PRESSURE
IS EXPECTED TO TRACK ACROSS PENNSYLVANIA SUNDAY NIGHT WITH AN
ASSOCIATED COLD FRONT MOVING ACROSS THE MID ATLANTIC. A REINFORCING
FRONT WILL CROSS THE REGION MONDAY. HIGH PRESSURE WILL BUILD IN
BEHIND THE FRONT BRINGING COOLER AND DRIER CONDITIONS DURING MIDWEEK.

&&

.NEAR TERM /THROUGH SUNDAY/...
WE WILL BE WATCHING MULTIPLE SHORTWAVES EMBEDDED WITHIN
WEST/NORTHWEST FLOW ALOFT OVER THE NEXT 24 OR SO HOURS.

CLOSEST UPSTREAM SHORTWAVE TO AFFECT THE CWA WAS MOVING INTO THE
POTOMAC HIGHLANDS AT 01Z. CONVECTION WAS STRONGER WHEN IT WAS OVER
OH/WV AND HAS WEAKENED SOME OVER THE PAST FEW HOURS. MODELS
INCLUDING HIGH RESOLUTION GUIDANCE HAVE NOT BEEN ABLE TO HANDLE
NEAR TERM CONVECTION VERY WELL THIS EVENING. MOST RECENT RUNS OF
THE HRRR HAVE LATCHED ONTO UPSTREAM CONVECTION BUT HRRR HAS
CONSISTENTLY BEEN DISSIPATING CONVECTION WHICH HASN/T HAPPENED
YET. ENVIRONMENT IS NOT SUPPORTIVE FOR SEVERE WX AS IT WAS IN OH
/AND THE SUN HAS NOW SET/...YET SHORTWAVE AND WARM ADVECTION WILL
LIKELY BE ABLE TO MAINTAIN SOME CONVECTION FOR AT LEAST A COUPLE
MORE HOURS INTO AT LEAST WESTERN MARYLAND...THE EASTERN WEST
VIRGINIA PANHANDLE AND INTO THE SHENANDOAH VALLEY OF VIRGINIA.
WHILE SEVERE WX IS NOT EXPECTED...HEAVY DOWNPOURS...LIGHTNING AND
LOCALLY GUSTY WINDS MAY OCCUR. WHETHER OR NOT CONVECTION LASTS
EAST OF THE BLUE RIDGE IS UNCERTAIN. CURRENT FORECAST LOWERS POPS
AT THIS POINT AND WILL NEED TO CONTINUE TO MONITOR RADAR TRENDS IN
CASE LATE EVENING POPS NEED TO BE INCREASED FURTHER DOWNSTREAM.

IN THE WAKE OF THE AFOREMENTIONED SHORTWAVE/EVENING
CONVECTION...ANOTHER SHORTWAVE OVER THE OHIO VALLEY MAY PROMOTE
LATE NIGHT AND EARLY SUNDAY MORNING CONVECTION AND POPS RAMP UP
/OR RE-RAMP UP/ FROM WEST TO EAST DURING THIS TIME. GIVEN TIME OF
DAY...SEVERE WX IS NOT EXPECTED WITH THIS ROUND EITHER HOWEVER
LOCALLY STRONG STORMS CAN/T BE RULED OUT.

A THIRD AND POSSIBLY STRONGEST SHORTWAVE /ALONG WITH MULTIPLE LOW
LEVEL BOUNDARIES/ THEN LOOKS TO PRODUCE MORE SHOWERS AND
THUNDERSTORMS SUNDAY AFTERNOON AND EVENING. CLOUDS FROM POTENTIAL
EARLY MORNING CONVECTION MAY HINDER INSTABILITY SLIGHTLY...BUT
EXPECTATION IS FOR ENOUGH OF A WINDOW FOR BREAKS IN THE CLOUDS TO
OCCUR TO CONTRIBUTE TO 1-2K J/KG CAPE COINCIDENT WITH 0-6KM SHEAR
LIKELY IN EXCESS OF 40 KT. POTENTIAL EXISTS FOR A WIDESPREAD
AND/OR SIGNIFICANT SEVERE EVENT SUNDAY AFTERNOON AND EVENING WITH
ALL SEVERE HAZARD TYPES POSSIBLE INCLUDING DAMAGING WINDS...LARGE
HAIL...FLASH FLOODING AND ALSO ISOLATED TORANDOES.

[Kmlwx]

Another SREF run where best params are south of us. But look at this map for sigtor ingredients - not very often you see something like this...

[high risk]

     and outflow boundaries can also back the low-level winds much more than the ambient synoptic pattern would dictate.  This can be really important on days with strong speed shear but little directional shear.    With the synoptic speed shear in place, backed low-level winds near an outflow boundary can provide sufficient low-level directional shear for significant storm rotation.

 

The storms that move through tonight, if there are any, could leave outflow boundaries that hang around through tomorrow. Knowing where these are could help figure out where storms fire tomorrow as the boundary can act as a trigger for convection. 

[mikeeng92]

AFD from Wakefield, pretty bullish for them.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH TUESDAY/...

CHALLENGING FORECAST SUNDAY/SUNDAY NIGHT DUE TO MODEL DIFFERENCES

WRT TIMING OF CONVECTIVE COMPLEXES PROGGED TO MOVE SE AHEAD OF

STRONG COLD FRONT. NAM SUGGESTS TWO ROUNDS OF CONVECTION (LATE MORN

ERLY AFTN WITH A SECOND ONE IN THE EVENING WHILE GFS/SREF SHOW

SCT CONVECTION DEVELOPING ON LEE TROF BY ERLY AFTN THEN DRIFTING

IT SE BUT EXPANDING IN COVERAGE ACROSS THE NORTH BY EVENING AS

SECONDARY S/W AND COLD FRONT APPROACH FROM THE NW. SPC CONTINUES TO HAVE

ENTIRE FA OUTLOOKED IN SLIGHT RISK WITH A MDT RISK TO THE W OVER W VA.

PARAMETERS FAVOR A LARGE HAIL/DAMAGING WIND THREAT BUT A TORNADO

THREAT ALSO EXISTS IF LEADING SUPERCELLS CAN DEVELOP. STAY TUNED.

[Disc]

Another SREF run where best params are south of us. But look at this map for sigtor ingredients - not very often you see something like this...

Wow. That 60 bullseye isn't very far to my west at all. I personally don't remember seeing sigtor that high in the area it is before.

[yoda]

00z NAM still likes evening time to overnight for us... and yes... there is 1500 SBCAPE and MLCAPE at 06z at KIAD

[Ian]

0z looks better at least.

[EasternUSWX]

Late in the evening but, not too bad.

 

[Ian]

Lol. That's a little more than late in the evening.

[EasternUSWX]

Lol. That's a little more than late in the evening.

 

Sssh. lol

[Kmlwx]

Part of me thinks that the parameters will be fine up our way despite them looking pretty south on some of the guidance. Tho maybe we will still fail anyway. 

[yoda]

Lol. That's a little more than late in the evening.

00z NAM likes 8pm to 2am as our "threat period." Good enough SBCAPE and MLCAPE as well

[Ian]

00z NAM likes 8pm to 2am as our "threat period." Good enough SBCAPE and MLCAPE as well

I haven't examined closely but we almost never get/verify a significant threat in that time period.

[yoda]

I haven't examined closely but we almost never get/verify a significant threat in that time period.

April 27 2011 comes to mind... but that was an entirely different monster. Though there was a 27 mile EF-2 at 2am in western VA

[Kmlwx]

00z NAM likes 8pm to 2am as our "threat period." Good enough SBCAPE and MLCAPE as well

Remember that it's "modeled" CAPE tho - doesn't mean it'll pan out. Losing battle to hope for a threat in that time period. Our derecho night was a big exception with a ton of CAPE that had yet to be reduced by later in the evening. We are dealing with much lower CAPE numbers here. Gotta try to get it earlier or it'll probably be a pretty marginal threat even with good shear in place. 

[CheesyPoofs]

April 27 2011 comes to mind... but that was an entirely different monster

A completely, completely different situation of course, but re: severe in our area post-sunset, the June 2012 derecho is also an example.

[snowfan]

I'm just going to expect that this turns into a run of the mill line of storms that passes through during the evening. Anything else will be a bonus. 

[EasternUSWX]

Most bullish LWX AFD i've seen for some sprinkles expected. lol

[Kmlwx]

In order for us to get late evening or nighttime severe something substantially has to be working in our favor. CAPE doesn't look anything near extreme levels in this potential event. Shear is pretty good and probably enough to sustain some decent storms well past sunset - but not sure if the nighttime threat is going to be as juiced as the models might seem to indicate.

[Kmlwx]

I completely agree. The earlier in the evening the better. I'm still concerned about getting enough breaks in the clouds for good CAPE to build, but I think someone gets something good tomorrow. Something is going to break our way eventually, right? 

I'm hoping for scattered severe but will be "happy" if I get some good gusts and nice lightning.