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2022 Mid-Atlantic Severe Wx Thread (General Discussion Etc)


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Morning AFD update from LWX as of 1035am

NEAR TERM /THROUGH SATURDAY/...
Latest surface analysis show a quasi-stationary front stalled
near the VA/NC border with a cold front draped across the Great
Lakes. The stationary front will gradually lift north through
the day and into tonight and eventually interact/collide with
the approaching cold front.

Through the early afternoon, the local region is going to be
positioned between two shortwaves with subsidence. Guidance
continues to suggest showers and thunderstorms develop on the
lee-side pressure trough late this afternoon as a weak shortwave
moves through. These showers/thunderstorms then slowly advance
to the northeast, posing an isolated flood/flash flood threat
for any storms that train over the same area. Uncertainty in
coverage and placement of the lee-pressure trough and therefore
the thunderstorm placement is somewhat uncertain. Current
thinking is anywhere from the Shenandoah Valley to east of the
Blue Ridge. Any thunderstorm is capable of producing gusty
winds.

As the aforementioned frontal boundaries begin to merge nearby,
a line of convergence will develop overnight. A stronger
shortwave with low-level to surface based broad low pressure
will move over the frontal boundary this evening into the
overnight. This low will continue advancing along the front
through Saturday. PWs will be near record levels for the day,
with widespread 2-2.5" values (2-3 SA above climo). The WCL will
be high (above 4 km), with a high FZL indicating warm rain
processes will be dominant. Given the forecast low-level
forcing, the PWs in guidance seem realistic. There are two
variables leading to some uncertainty: 1) The amount of
instability (generally 100-750 J/kg) and 2) The spread in models
with where exacting the positioning of the front will be and
the amount of low-level and fgen forcing present with the low as
it moves nearby. Should the higher magnitudes of forcing be
realized, a higher-impact heavy rain event would be plausible,
however should the lower magnitudes of forcing come to fruition,
this may be more of a beneficial light to moderate soaking
rain.

The area of greatest concern remains over the largely
hydrophobic Baltimore/Washington Metro areas. FFG is lowest in
this area. Will continue to assess latest 12Z guidance before
making any headline considerations.
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8 minutes ago, AmericanWxFreak said:

Seems like we’re just seeing this precip max just waffle around. Someone’s going to get some trouble tonight it’s just a matter of where. 

That's typical. QPF forecasting for systems that have several mesoscale features will waffle around. Only thing I've noticed over the years is that we see the axis of heavy rain wind up a bit further north and east from where the short term guidance shows up.

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33 minutes ago, Eskimo Joe said:

Seems the meso guidance pushing PWATS close to 3", while the globals only around 2". Wonder if that's a resolution issue, or if the short term guidance is keying in on something the globals might be missing out on.

It has felt supper humid so I guess it makes since that the PWATS are so high. What is the record for them around this time of year 

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Flood Watch out...some spicy language:

Quote
DISTRICT OF COLUMBIA-FREDERICK MD-CARROLL-NORTHERN BALTIMORE-
SOUTHERN BALTIMORE-PRINCE GEORGES-ANNE ARUNDEL-NORTHWEST MONTGOMERY-
CENTRAL AND SOUTHEAST MONTGOMERY-NORTHWEST HOWARD-CENTRAL AND
SOUTHEAST HOWARD-NORTHWEST HARFORD-SOUTHEAST HARFORD-PRINCE
WILLIAM/MANASSAS/MANASSAS PARK-FAIRFAX-ARLINGTON/FALLS
CHURCH/ALEXANDRIA-WESTERN LOUDOUN-EASTERN LOUDOUN-
INCLUDING THE CITIES OF SILVER SPRING, SOUTH GATE, BOWIE, ARLINGTON, 
ANNAPOLIS, MANASSAS, ODENTON, WESTMINSTER, DAMASCUS, FALLS CHURCH, 
PURCELLVILLE, BETHESDA, CHANTILLY, BALLENGER CREEK, CENTREVILLE, 
STERLING, COLLEGE PARK, REISTERSTOWN, COLUMBIA, HERNDON, ALEXANDRIA, 
SEVERNA PARK, GREENBELT, GERMANTOWN, LEESBURG, MCLEAN, CAMP SPRINGS, 
ROCKVILLE, JARRETTSVILLE, FREDERICK, ARNOLD, DALE CITY, 
SUITLAND-SILVER HILL, LAKE RIDGE, GAITHERSBURG, SEVERN, RESTON, 
MONTCLAIR, BALTIMORE, ELDERSBURG, CLINTON, GLEN BURNIE, ASHBURN, 
ELLICOTT CITY, WOODBRIDGE, WASHINGTON, ANNANDALE, FRANCONIA, 
COCKEYSVILLE, LISBON, LAUREL, AND ABERDEEN
1253 PM EDT FRI JUL 8 2022

...FLOOD WATCH IN EFFECT FROM 11 PM EDT THIS EVENING THROUGH 
SATURDAY AFTERNOON...

* WHAT...FLASH FLOODING CAUSED BY EXCESSIVE RAINFALL IS POSSIBLE.

* WHERE...WASHINGTON DC, MARYLAND AND NORTHERN VIRGINIA, INCLUDING 
  THE FOLLOWING AREAS: IN DC, DISTRICT OF COLUMBIA. IN MARYLAND, 
  ANNE ARUNDEL, CARROLL, CENTRAL AND SOUTHEAST HOWARD, CENTRAL AND 
  SOUTHEAST MONTGOMERY, FREDERICK MD, NORTHERN BALTIMORE, NORTHWEST 
  HARFORD, NORTHWEST HOWARD, NORTHWEST MONTGOMERY, PRINCE GEORGES, 
  SOUTHEAST HARFORD AND SOUTHERN BALTIMORE. IN NORTHERN VIRGINIA, 
  ARLINGTON/FALLS CHURCH/ALEXANDRIA, EASTERN LOUDOUN, FAIRFAX, 
  PRINCE WILLIAM/MANASSAS/MANASSAS PARK AND WESTERN LOUDOUN.

* WHEN...FROM 11 PM EDT THIS EVENING THROUGH SATURDAY AFTERNOON.

* IMPACTS...EXCESSIVE RUNOFF MAY RESULT IN FLOODING OF RIVERS, 
  CREEKS, STREAMS, AND OTHER LOW-LYING AND FLOOD-PRONE LOCATIONS.

* ADDITIONAL DETAILS...
  - SHOWERS ARE EXPECTED WITH SCATTERED THUNDERSTORMS LATE THIS
    EVENING INTO MIDDAY SATURDAY. AVERAGE RAINFALL AMOUNTS AROUND
    1 TO 3 INCHES ARE EXPECTED, BUT LOCALIZED AMOUNTS AROUND 4 TO
    7 INCHES IN HEAVIER SHOWERS AND THUNDERSTORMS. RAINFALL
    AMOUNTS AROUND 1 TO 3 INCHES WITHIN AN HOUR OR TWO ARE
    POSSIBLE IN AREAS WHERE THE HEAVIEST RAINFALL OCCURS. THE
    BEST CHANCE FOR THE HEAVIEST RAINFALL WILL BE OVERNIGHT INTO
    SATURDAY MORNING.
  - PLEASE VISIT HTTP://WWW.WEATHER.GOV/SAFETY/FLOOD FOR
    INFORMATION ON FLOOD SAFETY.

 

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^Gap is possible because:

1.) Terrain features can enhance flooding, plus they have lower FFG.

2.) Metro areas are rather vulnerable to flooding and have lower FFG.

Elsewhere, looks like guidance isn't hammering them much and they've been a bit drier. Just my two cents.

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WPC just placed a tiny bullseye of moderate risk for excessive rainfall right over the DC metro area:

...Ohio Valley, Central Appalachians, Mid-Atlantic
1930 UTC Update -- Per collaboration with WFO LWX, have hoisted a 
Moderate Risk area over parts of the Balt-Wash area for more 
widespread convection later tonight and into tomorrow morning 
(early portion of the Day 2 ERO period). There continues to be a 
multi-model heavy rainfall signal, especially from the CAMs, 
though the 18Z HRRR has broken a bit from the consensus by now 
showing the max QPF footprint a little farther south into Southern 
MD and the Northern Neck of VA. Will continue to keep an eye on 
observational and mesoanalysis trends this evening; for now have 
aligned the Moderate Risk area up with the general multi-model 
consensus, which also incorporates the more elevated 1 and 3 hour 
QPF exceedance probabilities per the 12Z HREF.

image.thumb.png.48000a6226e0d356cdc65722824e1e1b.png

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21 minutes ago, southmdwatcher said:

So this is going to be a southern MD/central VA event??.......well at least most of us have relatively dry ground

?? I think this is going to be a DC north into Baltimore event. It looks like a zone from 66 south to perhaps Charlottesville give or take will be the screw zone with the exception of some pockets of heavy amounts. 

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