Area Forecast Discussion
Issued by NWS Huntsville, AL

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000
FXUS64 KHUN 301140
AFDHUN

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HUNTSVILLE AL
540 AM CST FRI JAN 30 2015

.DISCUSSION...
WATER VAPOR ANIMATION SUGGESTS THAT THE MEAN LONGWAVE TROUGH POSITION
REMAINS ACROSS EASTERN NORTH AMERICA THIS MORNING...WITH AN
EMBEDDED VORT MAX CONTINUING TO DIG SOUTHEASTWARD ACROSS THE REGION.
WEAK ASCENT ASSOCIATED WITH THIS FEATURE HAS LED TO THE FORMATION OF
POCKETS OF LIGHT DRIZZLE EXTENDING FROM MIDDLE TENNESSEE INTO
NORTHEAST ALABAMA...AND WILL INCLUDE THIS IN THE FORECAST THROUGH
15Z. LOW STRATUS CLOUDS WILL LIKELY PERSIST THROUGH THE MORNING HOURS
FOR THE MAJORITY OF THE REGION...BEFORE INCREASING SUBSIDENCE IN WAKE
OF DEPARTING LONGWAVE TROUGH LEADS TO A MORE RAPID CLEARING TREND
FROM WEST-TO-EAST THIS AFTERNOON. THE COMBINATION OF PERSISTENT CLOUD
COVER AND A GUSTY NORTH-NORTHWEST WIND THIS AFTERNOON WILL LEAD TO
ONLY A MINOR INCREASE IN TEMPERATURES...WITH HIGHS EXPECTED TO REACH
THE LOWER/MID 40S. ALTHOUGH LOW CLOUDS SHOULD EVACUATE THE REGION BY
EARLY/MID AFTERNOON...CIRRUS WILL BE ADVECTED ACROSS THE REGION IN NW
FLOW ALOFT AS EARLY AS THIS EVENING. THIS...COUPLED WITH CENTER OF
SURFACE HIGH TRACKING EASTWARD THROUGH THE OHIO VALLEY WILL PREVENT
IDEAL RADIATIONAL COOLING...WITH LOW TEMPERATURES ONLY FALLING INTO
THE MID 20S. THESE VALUES WILL LIKELY BE ACHIEVED AROUND MIDNIGHT...
BEFORE CIRROSTRATUS CLOUDS THICKEN EARLY SATURDAY MORNING. THERE MAY
BE A BRIEF WINDOW OF OPPORTUNITY FOR THE FORMATION OF PATCHY FREEZING
FOG IN LOCAL VALLEYS LATE THIS EVENING...BUT DUE TO UNCERTAINTY WE
HAVE NOT INCLUDED THIS IN THE FORECAST.

BY SATURDAY...WINDS ALOFT WILL BACK TO THE WEST AS A CUTOFF UPPER LOW
ORGANIZING OVER SOUTHERN CALIFORNIA/NORTHERN BAJA LATER TODAY
BEGINS TO DROP SOUTH-SOUTHEASTWARD INTO NW MEXICO -- AND A STRONG
NORTHERN STREAM WAVE DIGS SOUTHEASTWARD ACROSS THE NORTHERN ROCKIES.
THIS REGIME WILL MAINTAIN AN OVERCAST COVERAGE OF THICK MID/HIGH-
LEVEL CLOUDS...WITH TEMPERATURES ONLY RISING INTO THE UPPER 40S/LOWER
50S. AS THE NORTHERN STREAM WAVE CONTINUES TO ADVANCE SOUTHEASTWARD
INTO THE NORTHERN PLAINS SATURDAY NIGHT...MID-LEVEL WINDS WILL BACK
TO THE SOUTHWEST AND INCREASE SETTING UP A STRONG VEERING/WAA PROFILE
AS INDICATED BY FORECAST SOUNDINGS. GLOBAL MODELS SUGGEST THAT LIFT
GENERATED BY ISENTROPIC ASCENT IN THE 280-300K LAYER WILL BECOME
RATHER STRONG -- ESPECIALLY BETWEEN 6-12Z SUNDAY -- AND WILL LIKELY
LEAD TO DEVELOPMENT OF LIGHT RAIN ACROSS THE REGION AS PWATS INCREASE
INTO THE 0.5-1 INCH RANGE. THE LIFT MAY BE ENHANCED FURTHER BY ONE OR
MORE LOWER AMPLITUDE DISTURBANCES EJECTING NORTHEASTWARD FROM THE NW
MEXICO UPPER LOW. DESPITE VERY DRY LOW-LEVEL AIR NOTED IN
SOUNDINGS...WE HAVE INCREASED POPS ABOVE MOS GUIDANCE AS MODELS
TYPICALLY UNDERESTIMATE THE MAGNITUDE OF ASCENT/SATURATION IN THESE
SCENARIOS. ALTHOUGH NOT STATED IN THE OFFICIAL FORECAST A FEW SLEET
PELLETS MAY MIX IN WITH RAIN AT THE INITIAL ONSET OF PRECIPITATION.
SHOWERS WILL SLOWLY INCREASE IN COVERAGE ON SUNDAY MORNING BEFORE
BECOMING FAIRLY WIDESPREAD ON SUNDAY AFTERNOON...AS A SURFACE LOW
ASSOCIATED WITH THE NORTHERN STREAM WAVE LIFTS NORTHEASTWARD FROM
EASTERN OKLAHOMA INTO CENTRAL KENTUCKY.

IN THE EXTENDED FORECAST...WE EXPECT WIDESPREAD SHOWERS TO BE ONGOING
ACROSS THE REGION SUNDAY EVENING...BEFORE MODIFIED ARCTIC AIR
INVADES ONCE AGAIN FROM THE NW. FRONTAL PASSAGE SHOULD OCCUR BETWEEN
00-06Z MONDAY...WITH A GUSTY NW WIND ADVECTING MUCH COLDER/DRIER AIR
SOUTHEASTWARD INTO THE REGION. THE THREAT FOR WIDESPREAD
PRECIPITATION WILL DIMINISH WITH PASSAGE OF FRONT...BUT MID-LEVEL
FORCING FOR UVM WILL REMAIN RATHER STRONG AS SOUTHERN PERIPHERY OF
THE NORTHERN STREAM WAVE DIGS SOUTHEASTWARD INTO THE REGION. THUS...
VERY LIGHT POSTFRONTAL PRECIPITATION IS STILL EXPECTED AND THIS WILL
MAINLY TAKE THE FORM OF A LIGHT SLEET/LIGHT SNOW MIXTURE. A FEW SNOW
FLURRIES WILL REMAIN A POSSIBILITY IN UPSLOPE FLOW REGIME TO THE EAST
OF I-65 BETWEEN 12-18Z MONDAY. OTHERWISE...EXPECT CLEARING SKIES BY
LATE AFTERNOON...WITH ANOTHER COLD MORNING ON TUESDAY AS LOWS DROP
INTO THE LOWER/MID 20S. THERE ARE STILL SIGNIFICANT TIMING
DIFFERENCES BETWEEN THE ECMWF AND GFS/GEM WRT TO EJECTION OF MEXICAN
CUTOFF LOW ON TUESDAY/WEDNESDAY. THE GENERAL CONSENSUS IS FOR THIS
FEATURE TO WEAKEN AND LIFT NORTHEASTWARD ALONG THE GULF COAST AND
INTO THE CAROLINAS...WITH PRECIPITATION ON NORTHERN PERIPHERY OF
ASSOCIATED SURFACE LOW POSSIBLY IMPACTING THE REGION WEDNESDAY
AFTERNOON/EVENING. TEMPERATURES NORTH OF THE TENNESSEE RIVER WOULD
SUPPORT A RAIN/SNOW MIX AND HAVE INCLUDED THIS IN THE FORECAST GRIDS.
ANOTHER ROUND OF SNOW MAY BE POSSIBLE ACROSS THE REGION ON
THURSDAY...ASSOCIATED WITH A SHARP VORT MAX DIGGING THROUGH BASE OF
THE LONGWAVE TROUGH. LONGER RANGE GUIDANCE SUGGESTS THE ARRIVAL OF AN
EVEN COLDER ARCTIC AIRMASS BY THURSDAY NIGHT/FRIDAY MORNING.

70/DD

&&

.AVIATION (FOR 12Z TAFS)...

VFR CONDITIONS ARE EXPECTED TO CONTINUE ACROSS N AL/S MIDDLE TN FOR
THE NEXT 24 HOURS. NORTHERLY WINDS OF 10-13KTS GUSTING TO 15-20KTS
ARE EXPECTED UNTIL 31/00Z WHEN WINDS SHOULD DROP TO 4KTS AND
VARIABLE.

SL.77

&&

.PRELIMINARY POINT TEMPS/POPS...
HUNTSVILLE    45  27  51  39 /  10   0   0  20
SHOALS        46  26  51  40 /   0   0   0  40
VINEMONT      46  25  52  40 /   0   0   0  30
FAYETTEVILLE  42  24  48  36 /  10   0   0  20
ALBERTVILLE   44  26  50  40 /  10   0   0  20
FORT PAYNE    44  25  50  37 /  10   0   0  10

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

$$

FOR MORE INFORMATION PLEASE VISIT OUR WEBSITE
AT WEATHER.GOV/HUNTSVILLE.


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