Area Forecast Discussion - Northern


FXAK69 PAFG 051132
AFDAFG

NORTHERN ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE FAIRBANKS AK
332 AM AKDT SAT SEP 5 2015
.DISCUSSION...
MAIN CONCERNS IN THE NEAR-TIME ARE FOG AND LOW-LEVEL STRATUS THIS
MORNING OVER THE NORTH SLOPE...YUKON DELTA...AND..MIDDLE AND
LOWER YUKON VALLEYS. ISOLATED-SCATTERED SHOWERS ARE ANTICIPATED
TODAY IN THOSE AREAS IN THE WEST COAST AND ALSO OVER THE SOUTHERN
SEWARD PENINSULA...NORTH SLOPES OF THE ALASKA RANGE...AND OVER THE
UPPER TANANA VALLEY. OTHERWISE...LOOKING AHEAD ATTENTION IS
TURNED TOWARDS THE NORTH SLOPE MON THROUGH WED AS A STRONG COLD
FRONT PUSHES SOUTH...FOLLOWED BY A STRONG DISTURBANCE.

FOG IN THE YUKON DELTA HAS DEGRADED VISIBILITIES SIGNIFICANTLY
EARLY THIS MORNING. AS SUCH ISSUED A DENSE FOG HEADLINE FOR ZONE
214. IN THE NORTH SLOPE...VISIBILITIES HAVE NOT BEEN OBSCURED BY
FOG AS ANTICIPATED...WITH VISIBILITY RANGING GENERALLY FROM A HALF
MILE TO 3 MILES OVER THE ARCTIC COAST.

OVERALL...00Z DETERMINISTIC MODELS WERE IN GOOD AGREEMENT THROUGH
THE FIRST 66-72 HRS. THE MEDIUM-RANGE RANGE MODELS ARE IN GENERAL
AGREEMENT IN BRINGING A TROUGH SOUTH INTO THE NORTHWEST COAST AND
ARCTIC COAST MON. HOWEVER...CAN SEE SOME DIFFERENCES IN AMPLITUDE
WITH REGARD TO THE DEVELOPMENT OF A LONG WAVE TROUGH OVER MAINLAND
AK AND A MID-LEVEL LOW IN THE BERING SEA MON-TUE. IN
ADDITION...DIFFERENCES IN AMPLITUDE AND PHASE ARE SEEN WITH
RESPECT TO A SHORTWAVE TROUGH THAT ROTATES THROUGH LATE TUE/EARLY
WED AND ITS EVOLUTION THEREAFTER.
.ALOFT...A RATHER QUIET 500 HPA PATTERN CURRENTLY IN PLACE WITH A
WEAK MID-LEVEL LOW CENTER EAST OF THE YUKON FLATS AND ANOTHER IN
THE CENTRAL BERING SEA. THE FORMER WILL CONTINUE TO DRIFT EAST
WHILE THE LATTER WILL MOVE INTO BRISTOL BAY OVERNIGHT TOMORROW
WHILE WEAKENING. THE AFOREMENTIONED WEAK LOW WILL CROSS THE ALASKA
PENINSULA AND MOVE INTO THE EASTERN GULF OF ALASKA EARLY MON. THE
LOW WILL CONTINUE TO MOVE EAST WEAKENING INTO AN OPEN WAVE EARLY
TUE OVER BRITISH COLUMBIA. 

AS MENTIONED EARLIER...MODEL FORECAST ARE IN GENERAL AGREEMENT IN
BRINGING A TROUGH AND ITS ASSOCIATED VORT LOBE SOUTH INTO THE
NORTHWEST COAST AND ARCTIC COAST MON. MEANWHILE...AS THE TROUGH
PUSHES SOUTH IT WILL COMBINE WITH A MID-LEVEL LOW IN THE WESTERN
BERING SEA LATE MON TO FORM A LONGWAVE TROUGH OVER NORTHWEST TWO-
THIRDS OF MAINLAND ALASKA EARLY TUE. THE ARCTIC TROUGH CONTINUES
TO BECOME MORE AMPLIFIED OVER MAINLAND ALASKA WITH ITS BASE MOVING
OVER THE ALASKA PENINSULA BY WED. A STRONG SHORTWAVE REINFORCES
THE ARCTIC TROUGH LATE TUE/EARLY WED THOUGH THERE ARE SOME TIMING
AND LOCATION DIFFERENCES BETWEEN THE 00Z MEDIUM-RANGE MODELS.

LIGHT FLOW ALOFT EXISTS IN THE LOWER LEVELS...850 AND 700
HPA...THROUGH THE FIRST 48-HRS OF THE FORECAST. AS SUCH...NO WINDS
TO POTENTIALLY TAP INTO TO MIX DOWN INTO THE SURFACE. 
.SURFACE...LOOSE AND BROAD MSLP OVER NORTHERN ALASKA. A WEAK 1003
HPA IS IN THE NORTHEAST GULF OF ALASKA AND THIS IS A WEAK SFC
EXTENDED NORTH SOUTH OF THE BROOKS RANGE. WEAK LOW PRESSURE WILL
PERSIST OVER THE MAINLAND WITH A WEAK PRESSURE GRADIENT IN PLACE
AS THIS LOW WEAKENS OVER THE NEXT 24 HRS. HIGH PRESSURE WILL MOVE
IN AND PREVAIL OVER THE WEST COAST AND THE ARCTIC COAST SUN. LOW
PRESSURE WILL DEVELOP BUT STAY IN THE HIGH ARCTIC ASSOCIATED WITH
THE TROUGH DIVING SOUTH IN THE ARCTIC AND NORTHWEST COASTS MON. 

A STRONGER LOW IS THEN EXPECTED DEVELOP AND MOVE SOUTH INTO THE
NEARSHORE WATERS BY TUE HOWEVER...DETERMINISTIC MODELS ARE SHOWING
APPRECIABLE DIFFERENCES IN THE MSLP PATTERN. SPECIFICALLY IN
REGARD TO HOW FAR THE LOW WILL HAVE PROGRESSED SOUTH TUE AND ITS
STRENGTH. OF NOTE...THE ECMWF STRENGTHENS THE LOW TO A GREATER
DEGREE THAN THE GFS AND IS SLOWER IN BRINGING THE LOW INTO THE
ARCTIC COAST...BY EARLY WED. ENOUGH UNCERTAINTY STILL EXISTS IN
REGARD TO THE SMALLER SCALE FEATURES AND THE IMPACTS THAT WILL
COME FORTH FROM THE UNSETTLED WEATHER. AT ANY RATE...THE
DETERMINISTIC MODELS AND ENSEMBLE GUIDANCE SHOW A CHANCE FOR A
STORM DEVELOPING AT THIS TIME PERIOD.

ISOLATED-SCATTERED SHOWERS ARE ANTICIPATED TODAY IN THE SOUTHERN
HALF OF THE WEST COAST...FROM THE SOUTHERN SEWARD PENINSULA
SOUTH...AND OVER PORTIONS OF THE NORTH SLOPES OF THE ALASKA RANGE
AND THE UPPER TANANA VALLEY. OTHERWISE...DRY CONDITIONS ARE
EXPECTED ACROSS MUCH OF THE FORECAST AREA TODAY. STRATUS AND
CHANCES FOR ISOLATED-SCATTERED SHOWERS WILL LINGER IN THE LOWER
AND MIDDLE YUKON VALLEYS AS THE WEAK MID-LEVEL LOW MOVES SOUTHEAST
INTO THE GULF OF ALASKA MIDDAY SUN. INCREASING CHANCES OF PRECIP
EXPECTED OVER THE NORTH SLOPE AFTER MIDNIGHT TONIGHT...SUN...AND
SUN NIGHT. INCREASING CHANCES SPREADING TO THE REST OF THE WEST
COAST MON AS THE MID-LEVEL TROUGH DIVES SOUTH INTO THE NORTHWEST
AND ARCTIC COASTS. ELSEWHERE...THE POTENTIAL FOR MODERATE TO HEAVY
SNOW WILL EXIST ACROSS THE BROOKS RANGE AS THE AFOREMENTIONED TROUGH
PUSHES EAST LATE MON/EARLY TUE.
.COASTAL HAZARD POTENTIAL DAYS 3 AND 4...LOW.
WILL CONTINUE TO MONITOR FOR THE POTENTIAL FOR MODERATE TO STRONG
NORTH-NORTHWEST WINDS ACROSS THE WESTERN AND CENTRAL ARCTIC COASTS
TUE-WED.

&&
.FIRE WEATHER...NO FIRE WEATHER CONCERNS. MINIMUM RELATIVE
HUMIDITY VALUES WILL REMAIN ABOVE 40 PERCENT THROUGH THE FORECAST
PERIOD. 

&&
.HYDROLOGY...NO HYDROLOGY CONCERNS. RIVER LEVELS CONTINUE TO FALL
IN THE CHENA RIVER...SALCHA...AND CHATANIKA BASINS. 

&&

.AFG WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY FOR PKZ235-PKZ240-PKZ245.
&&

$$

ET SEP 15