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324 
FXUS62 KRAH 191540
AFDRAH

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC
1040 AM EST SAT JAN 19 2008

.SYNOPSIS...

LOW PRESSURE IN THE CENTRAL GULF OF MEXICO WILL MOVE EAST
NORTHEAST ACROSS FLORIDA THIS AFTERNOON AND JUST OFFSHORE THE
SOUTHEAST US COAST TONIGHT. MEANWHILE... ARCTIC HIGH PRESSURE WILL
BUILD EASTWARD AND SPREAD A VERY COLD AIR MASS INTO OUR REGION
TONIGHT THROUGH SUNDAY NIGHT.

&&

.NEAR TERM /UNTIL 7 PM THIS EVENING/...
AS OF 1030 AM SATURDAY... A VERY DIFFICULT FORECAST CONTINUES THROUGH
THIS EVENING. 12Z PROXIMITY SOUNDINGS CAME IN ON AVERAGE 3 TO 6
METERS HIGHER WITH 1000-850 AND 850-700 MB THICKNESSES THAN
PROGGED BY THE MODELS. IN ADDITION... NAM AND GFS BUFR SOUNDINGS
WERE TOO COOL IN THE LOWEST 5 K FEET RELATIVE TO THE OBSERVED 06
AND 12Z KGSO SOUNDINGS AND TOO COOL OVER RDU RELATIVE TO AIRCRAFT
SOUNDINGS BETWEEN 11 AND 13Z. THIS OBVIOUSLY WILL MEAN A SOMEWHAT
DELAYED ARRIVAL OF THE COLDER AIR FROM THE WEST. HOWEVER... THE
COLDER AND DRIER AIR IS ON THE MOVE... AS THE OBSERVED SOUNDINGS
DISPLAY INCREASINGLY BACKING WINDS IN THE LOW LAYERS... WHICH IS
INDICATIVE OF THE COLD ADVECTION IN THAT LAYER. NOT SURPRISINGLY
THE TEMPERATURE PROFILE HAS COOLED WITH TIME IN THE PAST 6-9
HOURS. FURTHERMORE... DEWPOINTS OVER SOUTHWESTERN VA AND NORTHWEST
NC ARE DROPPING INTO THE TEENS... WHICH IS INDICATIVE OF THE LOW
LEVEL COOL... DRY ADVECTION... AND THE POTENTIAL FOR FURTHER
NEAR SURFACE COOLING POTENTIAL VIA EVAPORATION. TOO MUCH DRYING
AND SUBSATURATION IN THE LOW LEVELS WOULD OBVIOUSLY CUT DOWN ON
QPF IN THE TRIAD AREA.

SPEAKING OF QPF... THE SOUTHWEST TO NORTHEAST ORIENTED
PRECIPITATION BAND THAT MOVED INTO CENTRAL NC OVERNIGHT APPEARED
TO BE CLOSELY TIED TO H85-7 FRONTOGENESIS RESULTING FROM THE
TROUGH IN THAT LAYER LOCATED OVER THE FOOTHILLS AND WESTERN
PIEDMONT OF NC. REGI0NAL RADAR MOSAIC EARLIER THIS MORNING SHOWED
AN EROSION OF THE NORTHERN EDGE OF THE PRECIPITATION SHIELD OVER
NORTHERN GA... EASTERN TN... AND WESTERN NC DURING THE PAST
SEVERAL HOURS. THIS WAS APPARENTLY IN RESPONSE TO SUBSIDENCE AND
DNVA IN THE WAKE OF A MID LEVEL SHEAR VORT MAX (NORTH OF AN 80 KT
MID LEVEL JET STRETCHING FROM KFFC TO KGSO PER 12/H5 RAOB
ANALYSIS)... UPSTREAM OVER THE UPSTATE OF SC. THE MODELS CONTINUE
TO TREND THE PRECIPITATION SLIGHTLY FURTHER SOUTHEAST AND
POTENTIALLY KEEP THE NORTHWEST PIEDMONT RELATIVELY DRY AND SHORT
OF ADVISORY CRITERIA.... ESPCIALLY GIVEN VEERING LOW LEVEL WINDS
AND INCREASING DOWNSLOPE COMPONENT. IN FACT... SOME CLEARING IS
NOTED ON THE LATEST VISIBILE IMAGERY IN THE LEE OF THE
MOUNTAINS... WHERE KINT RECENTLY WENT TO FEW050. HOWEVER... WHERE IT IS
PRECIPITATING UPSTREAM... THE MODELS ARE UNDERDOING THE AMOUNT OF
LIQUID EQUIVALENT BY 25 TO 100 PERCENT. IN ADDITION... AN AREA OF
ENHANCED PRECIPITATION WAS EXPANDING OVER CENTRAL AL AND WEST
CENTRAL GA PER THE LATEST REGIONAL RADAR MOSAIC... AND THIS AREA
APPEARS RELATED TO 1.) MID LEVEL FGEN BETWEEN H7-5 AND BENEATH THE
RIGHT ENTRANCE OF THE UPPER JET... 2.) ANOTHER MID LEVEL
PERTURBATION OVER SW AL... AND 3.) AN ENHANCED OVERRUNNING LOW
LEVEL JET EAST OF THE DEEP CONVECTION AND SUBSEQUENT LATENT
HEATING OVER THE GULF. PURE LINEAR EXTRAPOLATION OF THIS
PRECIPITATION AREA WOULD GRAZE THE NORTHWEST PIEDMONT. AM
CONCERNED THAT THE INCREASINGLY ELEVATED AND WEAK LIFT/OMEGA WITH
WESTWARD EXTENT OVER THE CWA... VIA VIA FGEN AND DYNAMICS FROM THE
DEAMPLIFYING UPPER WAVE WILL BE INSUFFICIENT TO PRODUCE ADVISORY
CRITERIA IN THE NORTHWEST PIEDMONT AND WARNING CRITERIA IN
ALAMANCE AND PERSON COUNTIES. HOWEVER... THE FOLLOWING REASONING
SUPPORTS A LOW CONFIDENCE CONTINUANCE OF THE ADVISORY IN THESE
AREAS... 1.) THE MODELS POOR PERFORMANCE WITH PRECIP AMOUNTS
UPSTREAM... 2.) THE FACT THAT THE COLD AIR IS SHOWING SIGNS OF
INFILTRATING THESE AREAS AND SNOW RATIOS WILL ACCORDINGLY INCREASE
CLOSER TO 9 OR 10 TO 1... AND 3.) FORCING FOR ASCENT... ALBEIT
INCREASINGLY ELEVATED... IS EXPECTED TO CONTINUE INTO LATE THIS
AFTERNOON IN THESE AREAS. WOULD EXPECT THAT IF ADVISORY CRITERIA
IS MET THAT IT WOULD BE OVER RANDOLPH... SOUTHEASTERN GUILFORD AND
SOUTHERN DAVIDSON COUNTIES... WITH PERHAPS ONLY A DUSTING (IF THAT)
TOWARD EXTREME NORTHWESTERN FORSYTH. FEEL PRETTY CONFIDENT THAT
WARNING CRITERIA WILL NOT BE MET IN PERSON OR ALAMANCE EITHER...
BUT WILL LET THESE AREAS RIDE GIVEN THE UNCERTAINTY OF HOW THE
PRECIPITATION UPSTREAM WILL EVOLVE... AND ITS KIND OF SILLY TO
DOWNGRADE JUST THESE TWO COUNTIES TO AN ADVISORY.

ELSEWHERE IN THE WARNING AND ADVISORIES... THE FORECAST RATIONALE
AND RESULTANT SNOWFALL AMOUNTS WILL LIKELY BE RELATIVELY SIMILAR
TO THE PREVIOUS FORECAST PACKAGE... THOUGH LIKELY ON THE LOWER
END OF THE RANGE. EXPECT A SWEET SPOT ON EITHER SIDE OF A LINE STRETCHING
FROM PERHAPS EASTERN WAKE NORTHEASTWARD TO LOUISBURG AND ROANOKE
RAPIDS THAT WILL RECEIVE THE HIGHEST TOTALS.. WHERE THE ARRIVAL OF
THE COLDER AIR WILL CO-EXIST WITH THE HIGHER QPF. MOST
ACCUMULATION WILL OCCUR ON GRASSY SURFACES... CARS... AND ROOFTOPS
GIVEN THE ABOVE FREEZING NEAR SURFACE TEMPERATURES AND ALREADY WET
GROUND. THUS ROADWAYS.. SHOULD MAINLY BE WET OR SLUSHY THROUGH
SUNSET... WITH A HARD FREEZE OVER WHEN TEMPERATURES DROP INTO THE
20S AFTER SUNSET.  -MWS



&&

.SHORT TERM /7 PM THIS EVENING THROUGH 6 PM SUNDAY/...
LIGHT SNOW WILL CONTINUE THIS EVENING ACROSS THE EASTERN HALF OF
THE AREA...QUICKLY PUSHING EAST AS THE COASTAL STORM RAPIDLY MOVES
FURTHER OFFSHORE. MOST PRECIP SHOULD BE GONE BY 06Z. AFTER
THAT...ARCTIC HIGH PRESSURE BEGINS TO BUILD IN AND OUR
TEMPERATURES WILL QUICKLY DROP OVERNIGHT. LOWS IN THE LOW TO MID
20S EXPECTED. THIS COULD PRESENT A PROBLEM ON ROADWAYS WHICH WILL
BE PRONE TO ICING OVER.

SUNDAY LOOKS TO BE VERY COLD. LOW LEVEL THICKNESSES DO NOT CRACK
1250M. GUIDANCE APPEARS TO WARM AT THIS POINT SO WILL GO UNDER
GUIDANCE A FEW DEGREES. LOW TO MID 30S EXPECTED FOR HIGHS.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
AS OF 2 AM SATURDAY... COLDEST NIGHT DURING THIS ARCTIC OUTBREAK
TO OCCUR SUNDAY NIGHT/MONDAY MORNING...WITH MODEL CONSENSUS
INDICATING OPTIMAL RADIATIONAL CONDITIONS AS COLD SURFACE HIGH
BUILDS DIRECTLY OVER THE AREA...BEFORE PUSHING OFFSHORE DURING THE
DAY ON MONDAY. CURRENT THICKNESS FORECAST SUGGEST MID TEENS ACROSS
THE NORTH MONDAY MORNING. MODERATION BEGINS MONDAY WITH HIGHS 40
TO 45.

SURFACE TROUGH FORECAST TO MOVE INTO CENTRAL NORTH CAROLINA FROM THE 
NORTHWEST TUESDAY. TIMING AND THICKNESS FORECAST SUGGEST A RAIN 
EVENT. HIGH PRESSURE CONTROLS THE WEATHER INTO TUESDAY NIGHT. THE 
OPERATIONAL GFS IS SUGGESTING ANOTHER EVENT SIMILAR IN NATURE TO 
SATURDAY FOR THURSDAY TWENTY-FOURTH. CONFIDENCE IS EXTREMELY LOW AND 
WILL LEAN MORE TOWARD THE DRIER ECMWF MODEL AT THIS TIME. SOME 
INDICATIONS OF RETURNING MOISTURE FOR A CHANCE FOR RAIN BY FRIDAY... 
BUT THIS IS FAR ENOUGH OUT IN TIME TO ADVISE KEEPING RAIN CHANCES 
LOW.

ONCE SURFACE HIGH PRESSURE STARTS TO SLIP EAST MONDAY...TEMPERATURES 
BEGIN TO MODERATE TOWARD NORMAL FOR THE REMAINDER OF THE PERIOD. 
HIGHS IN THE 40S WITH LOWS 25 TO 35.

.AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
AS OF 915 AM SATURDAY... 
MVFR CIGS...PRIMARILY IN FOG ONGOING ACROSS CWA THIS MORNING. AREA 
OF MAINLY LIGHT RAIN WITH CIGS IN THE 3 TO 5 THOUSAND FOOT RANGE 
STRETCHES FORM SC INTO FAR SOUTHERN NC AND THEN INTO THE EASTERN 
PIEDMONT...WESTERN COASTAL PLAIN AND SANDHILLS REGION 
(RDU...FAY...RWI). EXPECT THE AREA OF PRECIP TO SPREAD N INTO THE 
WESTERN PIEDMONT THIS MORNING ARRIVING INTO GSO AND INT TOWARD NOON. 

LATEST AMDAR AIRCRAFT SOUNDINGS AT RDU (1219Z) SHOW FREEZING LEVEL 
DOWN TO AROUND 3700 FEET WITH A NEAR FREEZING ISOTHERMAL LAYER 
ABOVE. EXPERIMENTAL MMRR DATA NEAR IGX SHOWED A DROPPING FREEZING 
LEVEL EARLIER THIS MORNING. WITH A WARM LAYER NEAR THE SFC...PRECIP 
SHOULD REMAIN RAIN FOR THE NEXT SEVERAL HOURS AND THEN BEGIN TO MIX 
WITH SNOW ACROSS THE TRIANGLE AREA W AROUND OR JUST BEFORE MIDDAY. 
WITH THE ARRIVAL OF STEADIER PRECIP...CIGS SHOULD FALL INTO THE 
IFR/MVFR RANGE AND VSBYS SHOULD HOLD IN THE 1 TO 3 MILE RANGE. 

PRECIP WILL GRADUALLY CHANGE TO SNOW DURING THE AFTERNOON AND EARLY 
EVENING FROM NW TO SE. ALL OF THE PRECIP SHOULD TAPER OFF CLOSE TO 
SUNSET IN THE TRIAD...AROUND 9PM NEAR RDU AND TOWARD MIDNIGHT IN 
FAY. RAPID DRYING WILL MOVE OVERNIGHT AND SUNDAY SHOULD FEATURE VFR 
CONDITIONS WITH A NW FLOW. 

AS THE HIGH PRESSURE AREA MOVES OFF THE NORTHEAST COAST TUESDAY... A 
MOIST SOUTHWEST FLOW WILL LEAD TO INCREASING MOISTURE AND A LOWERING 
OF CLOUD BASES. MVFR CONDITIONS WITH SCATTERED SHOWERS WILL BE 
POSSIBLE TUESDAY AND TUESDAY NIGHT. HIGH PRESSURE WILL BUILD BACK 
OVER THE AREA WEDNESDAY.

&&


.RAH WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY FROM 4 PM THIS AFTERNOON TO 6 AM EST 
SUNDAY FOR NCZ077-078-085-086-088-089.
WINTER WEATHER ADVISORY UNTIL MIDNIGHT EST TONIGHT FOR 
NCZ073>075-083-084.
WINTER STORM WARNING UNTIL MIDNIGHT EST TONIGHT FOR NCZ007>010-
023>026-040-041-076.
WINTER WEATHER ADVISORY UNTIL 9 PM EST THIS EVENING FOR NCZ021-
022-038-039.
WINTER STORM WARNING FROM 4 PM THIS AFTERNOON TO 6 AM EST 
SUNDAY FOR NCZ011-027-028-042-043.

&&

$$
SYNOPSIS...MWS
NEAR TERM...MWS
SHORT TERM...RLH
LONG TERM...RLH
AVIATION...JFB