516  
FXUS61 KRNK 151447  
AFDRNK  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE BLACKSBURG VA  
947 AM EST FRI DEC 15 2017  
   
SYNOPSIS
 
 
A WEAK DISTURBANCE WILL PASS OVER THE NORTHERN MID-ATLANTIC  
WITH THE POTENTIAL FOR LIGHT WINTRY PRECIPITATION FROM NORTHERN  
VIRGINIA INTO PENNSYLVANIA. HIGH PRESSURE THEN RETURNS TONIGHT  
WITH BLUSTERY NORTHWEST WINDS, FOLLOWED BY FAIR WEATHER  
SATURDAY INTO EARLY SUNDAY.  
 

 
   
NEAR TERM /THROUGH TONIGHT/
 
 
AS OF 946 AM EST FRIDAY...INCREASED SKY COVER THROUGH THE MID-  
AFTERNOON HOURS, THESE CHANGES BEING MOST NOTICEABLE INTO NORTH  
CAROLINA. MANY AREAS THIS MORNING UNDER A OVC LOW DECK BETWEEN  
1000-2500' AGL, AS MOISTURE CONTINUES TO BE LOFTED NORTHEASTWARD  
ON THE HEELS OF A 80-KT SOUTHWESTERLY 500 MB JET PER REGIONAL  
DOPPLER WIND PROFILER DATA. GUIDANCE IS TRENDING LESS  
OPTIMISTICALLY ON SUNSHINE ANY SOONER THAN EARLY TO MID  
AFTERNOON. EXPECT ONLY A SLOW RISE IN TEMPERATURES WITH LITTLE  
SUNSHINE TODAY. THOUGH THE NAM CONTINUES TO OVEREXAGGERATE ITS  
FORECAST QPF GIVEN THE SUB-CLOUD AIR MASS IS RATHER DRY, DID  
HEAR OF A REPORT OF "SNIZZLE" NEAR LEWISBURG RECENTLY SO WILL  
LEAVE POPS/WX AS IS FOR NOW.  
 
PREVIOUS DISCUSSION ISSUED AT 400 AM FOLLOWS...  
 
BROAD NORTHERN STREAM TROUGH OVER THE EASTERN U.S. WILL EJECT  
ONE MORE SHORT WAVE THIS FORECAST PERIOD BEFORE THE ENTIRE  
PATTERN CHANGES THIS WEEKEND. THE FINAL SHORTWAVE IS WEAK AND  
PRETTY STRUNG OUT IN TERMS OF VORTICITY. DYNAMIC LIFT IS WEAK,  
THUS RELYING ON ISENTROPIC LIFT IN THE LOW LEVELS TO GENERATE  
ANY PRECIP, AND THAT ALONE IS PRETTY ANEMIC. THE ACTUAL SURFACE  
FRONT WHICH PASSED THROUGH THE FORECAST AREA YESTERDAY HAS  
STALLED OVER THE EASTERN CAROLINAS. AS THE UPPER DISTURBANCE  
SWINGS THROUGH THIS MORNING, A SURFACE LOW IS FORECAST TO  
DEVELOP ALONG THE OLD FRONTAL BOUNDARY OVER EASTERN NORTH  
CAROLINA THEN MOVE NORTHEAST OFF THE MID-ATLANTIC COAST.  
 
THE AIRMASS NORTH OF THE BOUNDARY AND DEVELOPING LOW PRESSURE IS  
QUITE DRY, SO NOT EXPECTING MUCH IN THE WAY OF PRECIP. INITIALLY  
THE LIFT WILL GO INTO THE DEVELOPMENT CLOUD COVER WITH CLOUD  
BASES OF 3-5KFT. ONLY ELEVATIONS ABOVE 3KFT IN VA AND EASTERN WV  
WOULD BE SUBJECT TO ANY RIME ICE AND OR VERY LIGHT SNIZZLE  
(FREEZING DRIZZLE AND OR ICE CRYSTALS (VERY LIGHT SNOW)) THIS  
MORNING...AND THIS WOULD BE PRIMARILY ALONG AND NORTH OF  
INTERSTATE 64 WHERE MODELS INDICATE SLIGHTLY DEEPER MOISTURE  
PROFILES. FARTHER SOUTH THE LOWER CLOUDS HAVE BEEN STRUGGLING TO  
DEVELOP. THE ONLY OTHER CLOUD ELEMENT VISIBLE IS COMING FROM  
THE CIRRUS SHIELD ACROSS THE SOUTHEAST WHICH IS FORECAST TO  
REMAIN PRIMARILY SOUTH OF THE VA/NC BORDER TODAY.  
 
FORECAST FOR OUR CWA WILL REFLECT SLIGHT CHANCE LIGHT SNOW OR  
LIGHT FREEZING DRIZZLE MAINLY NORTH OF THE ROANOKE VALLEY THIS  
MORNING, BUT CONFIDENCE VERY LOW THAT ANYTHING MATERIALIZES MUCH  
BELOW 3000 FT IN ELEVATION IN TERMS OF THE FREEZING DRIZZLE. IN  
ORDER TO GET PRECIP INTO THE VALLEYS, THE MOISTURE PROFILE  
WOULD HAVE TO GET DEEPER, BUT IF IT BECOMES TOO DEEP, IT WOULD  
FAVOR ICE CRYSTAL GROWTH VS. SUPERCOOLED WATER THUS PERMITTING  
VERY LIGHT SNOW INSTEAD OF DRIZZLE. EITHER WAY, IMPACT NOT  
MEANINGFUL AND WILL NOT INTRODUCE ANY ACCUMULATIONS OF ICE OR  
SNOW TO THE FORECAST. THE ONLY EXCEPTION MIGHT BE A DUSTING IN  
THE MOUNTAINS NORTH OF LEWISBURG WV AND VA HIGHLANDS NORTH OF  
COVINGTON, BUT THATS ABOUT IT.  
 
BY 2PM THIS AFTERNOON, THE UPPER LEVEL DISTURBANCE PASSES EAST  
OF THE AREA. THIS WILL ALLOW WINDS TO BECOME MORE WESTERLY,  
ENDING THE ISENTROPIC LIFT AND TRANSITION TO SOME UPSLOPE  
CLOUDS/FLURRIES IN THE WEST AND DOWNSLOPE CLEARING IN THE EAST.  
TEMPERATURES TODAY WILL STRUGGLE PER PRESENCE OF CLOUD COVER,  
REMAINING IN THE 30S. READINGS IN THE 40S MAY BE OBTAINED FROM  
THE PIEDMONT SOUTHWARD INTO NC WHERE CLOUDS WILL THIN FOR THE  
AFTERNOON. COLD AIR ADVECTION WILL BRING ONE MORE COLD NIGHT  
BEFORE THE PATTERN CHANGES SATURDAY. THE COLD AIR ADVECTION AND  
PRESSURE RISES WILL ALSO RESULT IN BLUSTERY CONDITIONS THIS  
EVENING AND OVERNIGHT WITH GUSTS OF 20-35 MPH.  
 

 
   
SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/
 
 
AS OF 415 AM EST FRIDAY...  
 
THE WEATHER PATTERN OVER THE CONTINENTAL U.S. IS FORECAST TO  
CHANGE THIS WEEKEND...THE BROAD EASTERN U.S. TROUGH EXITING THE  
REGION WITH FLOW BECOMING MORE ZONAL.  
 
HEIGHTS SLOWLY BUILD SATURDAY INTO SUNDAY AND THIS WILL BE  
REFLECTED IN A GRADUAL MODERATION OF THE TEMPERATURES. THERE IS  
AN UPPER DISTURBANCE THAT WILL MOVE NORTHEAST OUT OF TEXAS, BUT  
IT IS FORECAST TO BECOME WEAKER WITH TIME AS THE BUILDING  
HEIGHTS OVER THE SOUTHEAST CONUS DAMPEN THE EFFECTS OF THE SHORT  
WAVE. MODELS REMAIN INCONSISTENT IN JUST HOW MUCH MOISTURE WILL  
MAKE IT INTO THE AREA GIVEN WESTERLY FLOW AND RIDGING ALOFT,  
BUT IT IS LOOKING LIKE ANYTHING MEASURABLE AT THIS POINT WOULD  
BE A TENTH OF AN INCH OR LESS. AS SUCH WILL CONTINUE TO  
ADVERTISE JUST LOW SHOWER CHANCES INTO THE WESTERN PART OF THE  
CWA LATER SUNDAY AND CHANCE POPS MOST SECTIONS SUNDAY NIGHT.  
SHOULD BE WARM ENOUGH FOR ALL RAIN GIVEN LATE DAY ARRIVAL WITH  
TEMPS IN THE 40S SUNDAY, AND ONLY FALLING INTO THE 30S TO AROUND  
40 SUNDAY NIGHT.  
 

 
   
LONG TERM /MONDAY THROUGH THURSDAY/
 
 
AS OF 301 PM EST THURSDAY...  
 
APPEARS MILD PERIOD TO MAINTAIN ITSELF FOR MUCH OF NEXT WEEK AS  
FLATTER UPPER FLOW AROUND RIDGING ACROSS THE SOUTHEAST STATES  
REMAINS IN PLACE FOR MOST OF THE PERIOD. WILL SEE SOME DIGGING  
NORTHERN STREAM SHORTWAVE ENERGY MAKE FOR A BRIEF RETURN TO PASSING  
EASTERN TROFFINESS AROUND MIDWEEK BUT OVERALL WEAKER NOW WITH EACH  
RUN. OTHER THAN FOR MUCH WARMER TEMPERATURES, MAIN CONCERN WILL BE  
WITH SHOWER CHANCES THAT COULD LINGER INTO MONDAY, AND THEN PERHAPS  
EXPAND AGAIN TUESDAY WITH A RESIDUAL SHEARING UPPER SYSTEM FROM  
MEXICO THAT WILL ATTEMPT TO UNDERCUT THE RIDGE. GUIDANCE SPREAD  
REMAINS QUITE LARGE IN NORTHWARD EXTENT OF DEEPER MOISTURE INTO  
WESTERLY FLOW ALOFT AS WELL AS TIMING THROUGH TUESDAY NIGHT. APPEARS  
INITIAL SYSTEM MUCH WEAKER SO ONLY RUNNING WITH LOW POPS INTO MONDAY  
NIGHT WITH SOMEWHAT HIGHER CHANCES TUESDAY PENDING ADDED SUPPORT  
COMING OUT OF THE SOUTHWEST. SHOULD FINALLY SEE A COLD FRONT CROSS  
FROM THE NORTHWEST WEDNESDAY ALLOWING FOR DRIER BUT ONLY SLIGHTLY  
COOLER WEATHER BEFORE TEMPERATURES REBOUND AGAIN THURSDAY AS HIGH  
PRESSURE SLIDES OFFSHORE. HIGHS TO REBOUND TO WELL ABOVE NORMAL  
LEVELS FOR EARLY NEXT WEEK WITH MOSTLY 50S TO NEAR 60 EAST BEFORE  
COOLING SOME TO 40S/NEAR 50 MOUNTAINS TO 50-55 EAST THURSDAY.  
 

 
   
AVIATION /15Z FRIDAY THROUGH TUESDAY/
 
 
AS OF 525 AM EST FRIDAY...  
 
UPPER LEVEL DISTURBANCE PASSING ACROSS THE NORTHERN MID-ATLANTIC  
WILL RESULT IN MVFR CIGS FROM CENTRAL WV/VA NORTHWARD INTO PA.  
VFR CEILINGS WILL CONTINUE ELSEWHERE.  
 
SEVERAL MODELS SUGGEST THAT LOW CLOUDS/STRATUS AND POTENTIALLY  
LIGHT SNOW AND FREEZING DRIZZLE COULD DEVELOP ALONG ESPECIALLY  
NORTH OF INTERSTATE 64. MOISTURE IS VERY SHALLOW, SO CONFIDENCE  
IS LOW WITH RESPECT TO ANY MEANINGFUL ACCUMULATION OR ICING.  
ASIDE FOR LIGHT ICING THREAT, NO SIGNIFICANT WEATHER HAZARDS OF  
NOTE. PASSAGE OF THE UPPER TROUGH THIS AFTERNOON WILL BRING A  
PERIOD OF GUSTY WINDS, MAINLY FROM A WESTERLY DIRECTION WITH  
SURFACE GUSTS OF 20-30 KTS LIKELY VCNTY OF THE RIDGE CRESTS  
DEVELOPING AFTER 20Z/3PM.  
 
UPSLOPE CLOUDS WILL LINGER WEST OF THE BLUE RIDGE TONIGHT,  
DISPERSING SATURDAY WITH WIDESPREAD VFR ANTICIPATED SATURDAY  
INTO SUNDAY.  
 
MEDIUM CONFIDENCE IN CLOUD BASES AND VISIBILITIES THROUGH THE  
TAF VALID PERIOD. LOW CONFIDENCE ON ANY PRECIP. MEDIUM TO HIGH  
CONFIDENCE FOR WIND SPEED AND DIRECTION.  
 
EXTENDED DISCUSSION...  
 
GENERALLY VFR CONDITIONS ARE EXPECTED SATURDAY AND SUNDAY.  
A CHANCE FOR SUB-VFR CONDITIONS FROM A WEATHER SYSTEM FORECAST  
TO EMANATE FROM THE MID-MISSISSIPPI VALLEY MONDAY INTO TUESDAY.  
 

 
   
RNK WATCHES/WARNINGS/ADVISORIES
 
 
VA...NONE.  
NC...NONE.  
WV...NONE.  
 

 
 

 
SYNOPSIS...PM  
NEAR TERM...AL/PM  
SHORT TERM...JH/PM  
LONG TERM...JH  
AVIATION...PM  
 
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