Area Forecast Discussion
Issued by NWS Anchorage, AK

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FXAK68 PAFC 201317
AFDAFC

SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ANCHORAGE AK
517 AM AKDT MON OCT 20 2014

.ANALYSIS AND UPPER LEVELS...

NUMEROUS UPPER LOWS AND SHORTWAVE LOW AMPLITUDE DISTURBANCES ARE
DOMINATING THE UPPER AIR PATTERN ACROSS ALASKA THIS MORNING. THE
MAIN UPPER LOW CENTER IS IN THE VICINITY OF BRISTOL BAY...WITH TWO
BASAL SHORTWAVES TRACKING ALONG ITS PERIPHERY NEAR 50N...ONE SOUTH
OF ADAK AND ANOTHER SOUTH OF KODIAK. THESE TWO SHORTWAVES ARE
PROPAGATING ALONG A STRONG 160 KT NORTH PACIFIC JET
STREAM...FORMING ALONG A SHARP BAROCLINIC ZONE AIDED BY THE
ARCTIC AIR POURING INTO THE NORTH PACIFIC FROM THE CHUKCHI SEA.
THE FIRST SHORTWAVE TROUGH HAS THE MOST POTENTIAL ENERGY
ASSOCIATED WITH IT...AND IS EXPECTED TO DEEPEN INTO A STRONG NEAR
970 HPA LOW IN THE GULF AS IT INTERACTS WITH THE ARCTIC AIR MASS
DIVING SOUTH FROM THE BERING.

IN THE NORTH GULF OF ALASKA...A WARM FRONT/ELONGATED SURFACE LOW
IS TRACKING INTO PRINCE WILLIAM SOUND THIS MORNING...WITH A
SHARPENING UPPER LEVEL TROUGH AXIS EXTENDING FROM THE BRISTOL BAY
LOW. THIS TROUGH AXIS IS BEGINNING TO INTERACT WITH THE LOW LEVEL
BAROCLINIC ZONE...WITH EXTENSIVE CONVECTIVE SHOWERS FORMING ALONG
THE LOW LEVEL FRONT AROUND PRINCE WILLIAM SOUND. COOLING CLOUD
TOPS IN THE VICINITY OF THE KENAI PENINSULA AND NORTH CORRELATE
TO INCREASING MASS ASCENT AHEAD OF THE TROUGH AXIS AS THE UPPER
LEVEL JET BECOMES INCREASINGLY DIVERGENT.

&&

.MODEL DISCUSSION...

THE MODELS ARE IN EXCELLENT AGREEMENT WITH THE LARGE SCALE
PATTERN HEADING INTO LATE THIS WEEK...BUT REMAIN IN POORER
AGREEMENT WITH BOTH THE CURRENT LOW IN THE GULF...AND ANOTHER
STRONG LOW MOVING INTO THE GULF TUESDAY AND WEDNESDAY. THE ECMWF
HAS BEEN SUPERIOR IN SIMULATING THE CURRENT DEVELOPING WINTER
PRECIPITATION EVENT ACROSS ANCHORAGE AND PRINCE WILLIAM
SOUND...AND WILL BE UTILIZED FOR THE BEGINNING OF THE FORECAST.
THE CANADIAN GDPS IS FAVORED FOR THE DEVELOPING STRONG NORTH
PACIFIC LOW EXPECTED TO MOVE INTO THE GULF AS IT REPRESENTED A
GOOD MIDDLE GROUND IN STRENGTH AND TRACK BETWEEN THE DISPARATE
ECMWF AND GFS.

&&

.SHORT TERM FORECAST...

SOUTHCENTRAL ALASKA...
THE FIRST SIGNIFICANT WINTER EVENT OF THE SEASON IS UNFOLDING
ACROSS PORTIONS OF ANCHORAGE...TURNAGAIN ARM...AND PORTAGE VALLEY
THIS MORNING. HAVE ALREADY RECEIVED REPORTS OF 3 TO 4 INCHES IN
ANCHORAGE PROPER...AND UP TO 7 ALONG THE HILLSIDE. SNOW WILL
CONTINUE TO BE MODERATE TO HEAVY AT TIMES IN THE MORNING BEFORE
ACTIVITY TAPERS LATE MORNING THROUGH THE AFTERNOON. THERE IS A
DRASTIC CUTOFF IN PRECIPITATION NORTH OF ANCHORAGE PROPER...AND
THAT WILL LIKELY CONTINUE HEADING NORTH TOWARD THE MATANUSKA
VALLEY. LIGHTER SNOW SHOWERS WILL MOVE INTO PARTS OF THE COPPER
BASIN AND SUSITNA VALLEY LATE THIS MORNING AND AFTERNOON. LIGHT
SNOW WILL WRAP BACK AROUND THE MAIN UPPER LOW CENTER TOWARD THE
SOUTHERN KENAI PENINSULA...BUT WILL BE MUCH LIGHTER THAN THE
ACTIVITY OVER ANCHORAGE AND TURNAGAIN ARM. SHOWERS WILL DIMINISH
TUESDAY AS ANOTHER GULF LOW MOVES NORTH...SETTING UP A DRIER
OFFSHORE FLOW PATTERN INTO WEDNESDAY.

SOUTHWEST ALASKA...
SNOW AND LIGHT SNOW SHOWERS WILL IMPACT MUCH OF SOUTHWEST ALASKA
THROUGH TONIGHT AS AN UPPER LEVEL LOW ROTATES AROUND THE REGION.
SNOW WILL TAPER OFF EARLIEST FROM THE ALASKA RANGE HEADING
WEST...WITH SNOW LINGERING OVER THE SOUTHWEST COAST THROUGH MONDAY
NIGHT BEFORE TAPERING OFF. DRY OFFSHORE FLOW THEN SETS UP THROUGH
WEDNESDAY WITH INCREASING NORTH-NORTHWEST WINDS.

BERING SEA/ALEUTIANS...
RAIN AND SNOW SHOWERS WILL BECOME WIDESPREAD AS GUSTY NORTHERLY
FLOW DEVELOPS ACROSS THE CENTRAL AND EASTERN BERING SEA AND
ALEUTIANS THROUGH MOST OF WEDNESDAY BEFORE WEAK HIGH PRESSURE
BUILDS IN.

&&

.LONG TERM FORECAST...

ANOTHER LOW MOVING INTO THE GULF ON TUESDAY WILL CAUSE COLD AIR
TO SPREAD INTO THE SOUTHERN MAINLAND THROUGH MID WEEK...PRODUCING
BELOW NORMAL TEMPERATURES AND GUSTY GAP WINDS ALONG THE GULF
COAST. THIS HOWEVER APPEARS TO BE A SHORT LIVED EVENT...AS A LOW
SKIRTING THE EASTERN SIBERIAN COAST ON FRIDAY WILL BRING
INCREASED CLOUD COVER AND WARMER TEMPERATURES BY NEXT WEEKEND.
THE FRONT ASSOCIATED WITH THIS SYSTEM WILL BRING RAIN AND GUSTY
WINDS TO THE WESTERN ALEUTIANS STARTING THURSDAY....THEN SPREAD
ONTO THE MAINLAND BY THE WEEKEND. THE UPPER TROUGH FROM THIS
SYSTEM THEN LOOKS TO SLOWLY DISSIPATE OVER THE MAINLAND THROUGH
NEXT WEEKEND...LEAVING THE MAINLAND GENERALLY CLOUDY AND SHOWERY
WITH NEAR NORMAL TEMPERATURES.

-DEK

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...WINTER WEATHER ADVISORY 101.
MARINE...GALE 155 165 170 172 173 174 179 185.
FIRE WEATHER...NONE.

&&

$$

AHSENMACHER OCT 14



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