584
FXAK69 PAFG 180003
AFDAFG
NORTHERN ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE FAIRBANKS AK
403 PM AKDT THU SEP 17 2020
SYNOPSIS
A STRONG LOW IN SOUTHWEST ALASKA WILL MOVE TO THE SEWARD
PENINSULA TONIGHT AND THEN PERSIST THERE INTO SUN. THIS WILL
BRING HEAVY RAIN TO THE WEST COAST AND WESTERN INTERIOR TODAY
THROUGH SAT, WITH RAIN SPREADING TO THE NORTH SLOPE LATE TONIGHT
AND FRI. THIS STORM IS CAUSING STRONG SOUTHERLY CHINOOK WINDS OVER
THE ALASKA RANGE. THE CHINOOK WINDS WILL TURN SOUTHWEST AND
WEAKEN OVER THE CENTRAL INTERIOR TONIGHT, AND OVER THE EASTERN
INTERIOR FRI AS A COLD FRONT MOVES EAST. WITH THE CHINOOK ENDING,
RAIN WILL SPREAD TO THE CENTRAL INTERIOR TONIGHT, AND THEN SPREAD
EAST ACROSS THE EASTERN INTERIOR ON FRI.
A LOW DEVELOPING NEAR PRUDHOE BAY ON FRI WILL PERSIST INTO SAT,
THEN MOVE EAST SAT NIGHT AND SUN. THIS WILL PULL COLD AIR OVER THE
WESTERN NORTH SLOPE FRI NIGHT AND OVER THE REMAINDER OF THE NORTH
SLOPE ON SAT AND CAUSE RAIN TO MIX WITH SNOW FRI NIGHT AND CHANGE
TO ALL SNOW ON SAT. CAN EXPECT 2-4 INCHES OF SNOW OVER THE BROOKS
RANGE FROM FRI NIGHT INTO SAT. THIS LOW WILL ALSO CAUSE N-NW WINDS
20-25 KT AND ELEVATED SURF ALONG THE ARCTIC COAST NEAR UTQIAGVIK
FRI THROUGH SUN, WITH W WINDS 25G35 KT OVER THE ERN ARCTIC SAT
AND SUN.
THE COLD FRONT MOVING EAST ACROSS ALASKA FRI AND SAT WILL BRING
COOLER TEMPERATURES AND LOWER THE SNOW LEVEL TO BETWEEN 2000-3000
FEET ACROSS THE WEST COAST AND INTERIOR FRI NIGHT INTO SAT AND
REMAINING THERE INTO SUN. COULD SEE LIGHT SNOW IN THE ALASKA RANGE
AND OTHER TERRAIN ABOVE 3000 FT.
ALOFT...
THE LONG WAVE PATTERN CONSISTS OF A TROUGH OVER THE BERING SEA AD
WESTERN ALASKA, AND A RIDGE THAT EXTENDS FROM THE GULF OF
ALASKA NORTH OVER EASTERN ALASKA AND NW CANADA INTO THE ARCTIC
OCEAN NORTH OF ALASKA. A STRONG SHORT WAVE NOW OVER SW ALASKA WILL
NORTHEAST ACROSS ALASKA TONIGHT AND FRI, THEN MOVE NORTH AND EAST
OF ALASKA FRI PM. THIS HAS SET UP STRONG SOUTH MOIST FLOW NOW
OVER ALASKA, WITH STRONG CHINOOK WINDS OVER THE ALASKA RANGE. THIS
IS SHORT WAVE WILL CAUSE RAIN TONIGHT AND FRI OVER MOST OF NRN
ALASKA. THE CHINOOK WINDS ARE LIMITING RAIN IN THE SE INTERIOR
TONIGHT. THE CHINOOK WILL END FROM THE WEST TONIGHT AND FRI AS THE
SHORT WAVE MOVES NORTHEAST OF THE ALASKA RANGE. SOUTHWEST FLOW
TRAILING THE SHORT WAVE WILL PUSH RAIN EAST OVER MOST OF THE ERN
INTERIOR FRI AND FRI NIGHT. WEAK SOUTHWEST FLOW ALOFT WILL PERSIST
FROM SAT INTO NEXT WEEK, KEEPING MOST OF NRN ALASKA COOL AND DAMP
FROM SAT INTO NEXT WEEK.
850 MB TEMPERATURES RANGE FROM 0C ALONG THE COASTS TO +8C OVER THE
ERN INTERIOR. WILL SEE ALL AREAS COOL 4C THROUGH FRI, THEN COOL
ANOTHER 1-2C THROUGH SAT, THEN REMAIN THE SAME INTO SUN EXCEPT
COOLING ANOTHER 2C OVER THE NORTH SLOPE. SURFACE TEMPERATURES
SHOULD TREND COOLER ALL AREAS FRI, WITH SLIGHT COOLING SAT, THEN
STABLE INTO SUN EXPECT COOLER ON THE NORTH SLOPE.
SURFACE...
A STRONG LOW OVER SOUTHWEST ALASKA WILL MOVE TO THE SEWARD
PENINSULA AS A 985 MB LOW BY 4AM FRI AND THEN WEAKEN SLOWLY IN
PLACE THROUGH SUN. AN OCCLUDED FRONT EXTENDING FROM THIS LOW TO
UNALAKLEET WITH A COLD FRONT FROM UNALAKLEET TO DENALI TO COOK
INLET, AND A WARM FRONT FROM UNALAKLEET TO EAGLE. BY 4AM FRI THE
OCCLUDED AND WARM FRONT WILL MOVE TO THE SEWARD PEN TO SAGWON TO
OLD CROW, WITH A TRAILING COLD FRONT FROM SAGWON TO DENALI.
BY 4PM FRI THE OCCLUDED FRONT WILL LIE FROM KOTZEBUE TO A 990 MB
LOW DEVELOPING NEAR PRUDHOE BAY TO DEMARCATION POINT WITH A COLD
FRONT FROM DEMARCATION POINT TO DELTA JUNCTION. BY 4AM SAT THE
FRONT WILL MOVE TO NOME TO A 990 MB LOW JUST NORTH OF PRUDHOE BAY
THEN EAST, WITH THE FRONT DISSIPATING AFTER THAT.
HEAVY RAIN IS SPREADING NORTH WITH THE FRONT OVER THE WEST COAST
AND WESTERN/CENTRAL INTERIOR NOW AND WILL SPREAD TO THE NORTH
SLOPE LATE TONIGHT AND FRI, AND TO MUCH OF THE ERN INTERIOR FRI
NIGHT AND SAT. RAIN WILL CONTINUE IN THOSE AREAS THROUGH FRI
NIGHT AND THEN TAPER OFF TO SCATTERED SHOWERS ON SAT AND SUN.
EXPECT 0.50-1.50 INCH OF RAIN OVER THE WEST COAST, WRN AND CENTRAL
INTERIOR, AND THE NORTHERN INTERIOR AND NORTH SLOPE THROUGH FRI
NIGHT.
STRONG DOWNSLOPE WINDS OVER THE ALASKA RANGE WILL CONTINUE TONIGHT
KEEPING THE SE INTERIOR DRY, BUT THE DOWNSLOPE WILL DIMINISH FROM
THE WEST ON FRI BEHIND THE COLD FRONT, AND RAIN WILL SPREAD EAST
WITH THE COLD FRONT. SO WINDS WILL DECREASE OVER DENALI FRI AM,
OVER THE CENTRAL ALASKA RANGE BY NOON FRI, AND BY 4PM OVER THE ERN
ALASKA RANGE.
THE 990 MB LOW JUST NORTH OF PRUDHOE BAY ON FRIDAY, THEN MOVE TO
100 NM NORTHEAST OF PRUDHOE BAY AS A 995 MB LOW BY 4PM SAT, THEN
MOVE SLOWLY EAST. THIS WILL PULL COLD AIR OVER THE WESTERN NORTH
SLOPE FRI NIGHT AND OVER THE REMAINDER OF THE NORTH SLOPE ON SAT
AND CAUSE RAIN TO MIX WITH SNOW FRI NIGHT AND CHANGE TO ALL SNOW
ON SAT. CAN EXPECT 2-4 INCHES OF SNOW OVER THE BROOKS RANGE FROM
FRI NIGHT INTO SAT. THIS LOW WILL ALSO CAUSE N-NW WINDS 20-25 KT
AND ELEVATED SURF ALONG THE ARCTIC COAST NEAR UTQIAGVIK FRI
THROUGH SUN, WITH W WINDS 25G35 KT OVER THE ERN ARCTIC SAT AND
SUN.
DISCUSSION
MODELS INITIALIZE ABOUT 10-20 METERS TOO LOW ON 500 MB HEIGHTS AT
12Z ACROSS NORTHERN ALASKA SO THEY ARE SLIGHTLY WEAK ON THE
GRADIENT ALOFT OVER NRN ALASKA. OTHERWISE MODELS INITIALIZE WELL.
MODELS SHOW SIMILAR SOLUTIONS ALOFT THROUGH 10PM TODAY, THEN THE
NAM DEEPENS THE 500 MB LOW OVER WRN ALASKA WHILE THE GFS/ECMF AND
CANADIAN MODELS ALL KEEP THE 500 MB LOW AT ABOUT THE SAME DEPTH.
THERE IS NO DRAMATIC INPUT OF ENERGY TO DEEPEN THIS LOW SO WE
FAVOR THE ECMF/GFS AND CANADIAN SOLUTIONS, WHICH ARE VERY SIMILAR
THROUGH 4PM SAT, WITH FAIR AGREEMENT THROUGH 4PM SUN. SINCE THE
MODELS INITIALIZE
WITH PRECIPITATION, MODELS SHOW SIMILAR SOLUTIONS THROUGH 10 PM
TODAY WITH SLIGHT DIFFERENCES IN DETAILS. WILL USE A BLEND OF
MODELS THROUGH 10PM THIS EVENING, THEN USE A BLEND OF THE GFS AND
ECMF AFTER THAT. THE NAM SOLUTION ALOFT IS NOT FAVORED AFTER 10
PM TONIGHT SO THE NAM PRECIP WILL NOT BE USED AFTER 10 PM. MAIN
CAVEAT IS THAT SIGNIFICANT PRECIP NOW OCCURING OVER THE WRN
INTERIOR AND W COAST WILL SPREAD TO THE NORTHERN AND CENTRAL
INTERIOR THIS EVENING, AND TO THE NORTH SLOPE LATE TONIGHT AND
FRI. PRECIP WILL START TO SPREAD EAST ACROSS THE ERN INTERIOR FRI
PM AS THE DOWNSLOPE ENDS.
AT THE SURFACE AT 15Z, MODELS ALL VERIFY ARE 3-5 MB TOO WEAK ON
THE SURFACE LOW OVER BRISTOL BAY, BUT VERIFY WELL ON OTHER
FEATURES. MODELS SHOW SIMILAR SOLUTIONS THROUGH 4AM FRI, THEN THE
GFS AND ECMF KEEP THE SURFACE LOW DEVELOPING ON THE ARCTIC COAST
FURTHER EAST THAN THE NAM. SINCE WE ARE NOT USING THE NAM ALOFT,
WE WILL ALSO NOT USE THE NAM AT SURFACE AFTER 10 PM TONIGHT. WILL
USE A BLEND OF THE GFS AND ECMF AT THE SURFACE. SINCE THE LOW NOW
IN BRISTOL BAY IS DEEPER THAN MODELS INDICATE, WILL ADD 2-3 KT TO
WINDS MOST AREAS THROUGH TONIGHT TO ACCOUNT FOR THE LOW REMAINING
DEEPER THAN MODELS INDICATE. WILL ALSO ADD 5-10 KT TO CHINOOK
WINDS THROUGH TONIGHT TO ACCOUNT FOR TIGHTER GRADIENT ALOFT. ALL
MODELS DIVERGE ON LOW ALONG ARCTIC COAST WITH THE GFS MOVING THE
LOW EAST MORE QUICKLY THAN THE ECMF. SINCE THE ENTIRE PATTERN IS
BECOMING PROGRESSIVE WILL USE THE FASTER GFS SOLUTION, BUT LINGER
WINDS OVER THE ARCTIC COAST LONGER SAT NIGHT AND SUN THAN THE GFS
TO ACCOUNT FOR POSSIBILITY OF THE ECMF BEING CORRECT.
BOTTOM LINE FOR MODELS IS THAT WE WILL USE A BLEND OF THE
GFS/NAM/ECMF THROUGH 10 PM TONIGHT, THEN USE A BLEND OF THE GFS
AND ECMF. WILL BUMP WINDS UP 2-3 KT ABOVE MODELS THROUGH TONIGHT
TO ACCOUNT FOR STRONGER SURFACE LOW, AND BUMP UP WINDS 5-10KT
ABOVE MODELS OVER ALASKA RANGE THROUGH TONIGHT TO ACCOUNT FOR
STRONGER GRADIENT ALOFT. WILL MOVE WINDS EAST ALONG THE ARCTIC
COAST FASTER PER THE GFS ON SAT NIGHT AND SUN, BUT LINGER WINDS
OVER THE WRN ARCTIC SAT NIGHT AND SUN TO ACCOUNT FOR THAT
POSSIBILITY.
COASTAL HAZARD POTENTIAL DAYS 3 AND 4...
ELEVATED SURF ALONG THE ARCTIC COAST NEAR UTQIAGVIK FRI THROUGH
SAT NIGHT WITH N-NW WINDS 20-25KT.
ELEVATED SURF AGAIN UTQIAGVIK WEST MON AND TUE WITH WEST WINDS 25
KT.
HYDROLOGY
EXPECT RISES ALL RIVERS WEST OF FAIRBANKS AND OVER NORTH SLOPE
THROUGH THE WEEKEND, AND STEADY OR SLOWLY FALLING RIVERS OVER THE
INTERIOR FROM FAIRBANKS EAST THROUGH THE WEEKEND. NO FLOODING IS
EXPECTED. RAIN TODAY THROUGH FRI OF 0.25 TO 1.00 INCH OVER THE
WESTERN/CENTRAL INTERIOR AND WEST COAST. ANOTHER 1 INCH OF RAIN
FRI NIGHT AND SAT OVER THE SOUTH SLOPES OF THE BROOKS RANGE.
SIGNIFICANT PRECIP OVER THE NORTH SLOPE FRI INTO SAT BUT MUCH OF
IT WILL BE SNOW.
AFG WATCHES/WARNINGS/ADVISORIES
WIND ADVISORY FOR AKZ223-AKZ225-AKZ226.
SMALL CRAFT ADVISORY FOR PKZ200-PKZ210-PKZ220-PKZ225-PKZ230-
PKZ235-PKZ240-PKZ245.
JB SEP 20
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