Mesoscale Discussion 1802

Issued

Oct 20, 2016, 2:28 PM EDT (18:28 UTC)

Expired

Oct 20, 2016, 5:00 PM EDT

In force

3 hr

Issued by

the Storm Prediction Center

Chance of a watch

60%

The Pittsburgh radar (KPBZ), 83 km from the middle of the area, began a volume at 18:29 UTC, the scan nearest the moment this went out.

Open the radar archive at this moment

The archived warnings, watches and discussions in force at that minute are drawn over the scan, this one among them.

• Severe Thunderstorm Watch 499Oct 20, 3:45 PM EDT

975 
ACUS11 KWNS 201829
SWOMCD
SPC MCD 201828 
PAZ000-MDZ000-WVZ000-OHZ000-202100-

MESOSCALE DISCUSSION 1802
NWS STORM PREDICTION CENTER NORMAN OK
0128 PM CDT THU OCT 20 2016

AREAS AFFECTED...PARTS OF SOUTHEASTERN OHIO...NORTHERN WEST
VIRGINIA...AND WESTERN AND CENTRAL PENNSYLVANIA

CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE 

VALID 201828Z - 202100Z

PROBABILITY OF WATCH ISSUANCE...60 PERCENT

SUMMARY...THE THREAT FOR SEVERE WEATHER OF ALL TYPES IS INCREASING
OVER THE AREA. THERE IS SOME TORNADO RISK OVER ALL THE AREA BUT IS
HIGHEST OVER PORTIONS OF WESTERN AND CENTRAL PA. A WEATHER WATCH MAY
BE NEEDED IN THE NEXT FEW HOURS.

DISCUSSION...VISIBLE SATELLITE TRENDS INDICATE BOUNDARY-LAYER
CUMULUS IS INCREASING ALONG BOTH THE EASTERN PERIPHERY OF THE
WIDESPREAD CLOUD/PRECIPITATION SHIELD OVER THE OHIO VALLEY AND ALONG
A PRE-FRONTAL CONFLUENCE ZONE TO THE WEST OF THE ALLEGHENY MNTS OVER
SOUTHWESTERN PA.  A FEW SMALL ECHOES HAVE APPEARED OVER THE NORTHERN
PANHANDLE OF WEST VIRGINIA...AN INDICATION OF ERODING CINH. 

WIDESPREAD SURFACE HEATING HAS LED TO TEMPERATURES INCREASING INTO
THE UPPER 70S TO LOW 80S OVER THE WESTERN PART OF THE AREA...WITH
HEATING OVER THE EASTERN/NORTHERN PART OF THE AREA LAGGING A LITTLE
BEHIND AS THE WEDGE FRONT/LOW-LEVEL STRATUS HAVE BEEN SLOW TO ERODE.
CONSEQUENTLY...LOW-LEVEL LAPSE RATES HAVE BECOME FAIRLY STEEP /7.5
TO 8 DEG C PER KM/ OVER MUCH OF THE AREA. 

ALTHOUGH SHEAR PROFILES ARE LARGELY UNIDIRECTIONAL AT THE PRESENT
TIME /EXCEPT FOR A VERY SHALLOW SURFACE LAYER WITH SOUTHERLY WINDS
PER SURFACE OBS AND KCCX VAD WINDS/...RAP AND HRRR FORECASTS SUGGEST
THE HODOGRAPHS WILL RAPIDLY OBTAIN CLOCKWISE CURVATURE OVER THE NEXT
FEW HOURS AS THE SURFACE LOW DEEPENS AND APPROACHES THE AREA.
FURTHERMORE...THE EFFECTIVE BULK SHEAR -- CURRENTLY AROUND 
45-50 KT -- SHOULD INCREASE WITH TIME AS THE DEEPENING TROUGH/JET
STREAK TRANSLATE EASTWARD OVER THE AREA.

THE PRIMARY LIMITING FACTOR IS RATHER MODEST MID-LEVEL LAPSE RATES
AND THE ASSOCIATED MODEST MIXED-LAYER CAPE VALUES OF 250-750 J/KG.
THESE MLCAPE VALUES ARE EXPECTED TO RISE ONLY TO 1250 J/KG AT THE
HIGHEST. A LACK OF STEEPER MID-LEVEL LAPSE RATES IS PROVIDING SOME
UNCERTAINTY TO THE LONGEVITY/INTENSITY OF INDIVIDUAL CELLS THAT ARE
NOT BEING FORCED ALONG THE FRONT. HOWEVER...THE WIDESPREAD
HEATING...ERODING CINH...AND SHEAR PROFILES THAT WILL BECOME
INCREASINGLY FAVORABLE FOR ORGANIZED STORMS INCLUDING SEVERE WIND
GUSTS...SEVERE HAIL...AND A TORNADO OR TWO MAY REQUIRE A WATCH
ISSUANCE IN THE NEXT FEW HOURS.

..CONIGLIO/COHEN/DARROW.. 10/20/2016

...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT...

ATTN...WFO...CTP...LWX...PBZ...RLX...

LAT...LON   41917831 41977716 41597696 41087698 40177775 39607840
            39087942 38508023 38568132 38708195 39418198 40198119
            40958038 41917831

Mesoscale Discussions in West Virginia, 2016Mesoscale Discussions in Pennsylvania, 2016Mesoscale Discussions in Ohio, 2016Every mesoscale discussion in 2016

The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.