174  
FXAK68 PAFC 260037  
AFDAFC  
 
SOUTHCENTRAL AND SOUTHWEST ALASKA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE ANCHORAGE AK  
437 PM AKDT WED APR 25 2018  
   
ANALYSIS AND UPPER LEVELS
 
 
A POOL OF COLD AIR ALOFT FROM THE NEARLY STATIONARY LOW JUST SOUTH  
OF THE BERING STRAIT CONTINUES TO BRING WIDESPREAD RAIN AND SNOW  
SHOWERS AS WELL AS WIDESPREAD CLOUDS TO MUCH OF THE SOUTHERN  
ALASKA MAINLAND. THE MOST WIDESPREAD SHOWER ACTIVITY HAS OCCURRED  
OVER THE NORTHERN SUSITNA VALLEY WHERE PERSISTENT AND MOIST  
SOUTHWESTERLY FLOW HAS PRODUCED RAIN AND SNOW SINCE THE FRONT  
PASSED THROUGH THAT AREA YESTERDAY EVENING, ALLOWING FOR ABOUT  
4 INCHES OF SNOWFALL IN THE CANTWELL AREA SINCE YESTERDAY  
EVENING. OVER THE SOUTHWEST MAINLAND SHOWERS (MOSTLY IN THE FORM  
OF SNOW) CONTINUE TO MOVE THROUGH THE AREA AND ARE BEGINNING TO  
INTENSIFY WITH DAYTIME HEATING. THIS MAY ALLOW FOR THE DEVELOPMENT  
OF A FEW ISOLATED THUNDERSTORMS THIS EVENING (ESPECIALLY OVER  
EASTERN BRISTOL BAY). CONDITIONS FURTHER WEST OVER THE BERING  
SEA/ALEUTIANS ARE MUCH QUIETER BEHIND THE UPPER LEVEL TROUGH AS  
WEAK RIDGING IS ALLOWING FOR THE DEVELOPMENT OF STRATUS OVER THE  
WESTERN BERING AND DIMINISHING WINDS.  
 

 
   
MODEL DISCUSSION
 
 
THE MODELS REMAIN IN GREAT SYNOPTIC AGREEMENT THROUGH THE WORK  
WEEK WITH THE TIMING OF A SERIES OF WAVES THAT WILL ROTATE AROUND  
THE QUASI-STATIONARY UPPER LOW THIS WEEK. EVEN WITH THIS GOOD  
AGREEMENT, THERE CONTINUE TO BE DIFFERENCES IN THE MODELS IN THE  
PLACEMENT OF SHOWER ACTIVITY AS IS OFTEN THE CASE IN THESE  
SOUTHWESTERLY FLOW EVENTS. AS A RESULT, DETAILS IN THE PRECIPITATION  
FORECAST INTO FRIDAY FOCUS ON WHERE THERE IS AGREEMENT, SUCH AS  
UPSLOPE PRONE AREAS OF THE SUSITNA VALLEY AND GULF COAST, WITH  
WIDESPREAD CHANCE POP'S ELSEWHERE. LOOKING INTO FRIDAY, AGREEMENT  
BEGINS TO IMPROVE SIGNIFICANTLY AS A MUCH DEEPER UPPER LEVEL LOW  
MOVES THROUGH THE BERING INTO THE SOUTHERN MAINLAND. THIS IS WHERE  
MUCH OF THE FOCUS WAS FOR FORECAST UPDATES TODAY.  
 

 
   
AVIATION
 
 
PANC...GUSTY SOUTH TO SOUTHEASTERLY WINDS ARE EXPECTED TO  
CONTINUE INTO THURSDAY WITH LITTLE CHANGE THE PRESSURE GRADIENTS  
ALONG THE CHUGACH. THERE WILL BE A UPTICK IN THE WINDS THIS  
EVENING, PRODUCING GUSTS TO AROUND 35 KT, WHICH SHOULD THEN  
FALL TO AROUND 25 KT AFTER SUNSET AS DAYTIME HEATING DIMINISHES.  
OTHERWISE CONDITIONS SHOULD REMAIN MOSTLY VFR AS RAIN AND SHOW  
SHOWERS PASS THROUGH THE AREA. FORECAST CONFIDENCE IS HOWEVER  
LOW IN THE TIMING AND TRACK OF THESE SHOWERS, SO THERE IS A  
POSSIBILITY THAT CONDITIONS MAY INTERMITTENTLY DROP TO MVFR IF  
HEAVIER SHOWERS MOVE THROUGH THE VICINITY OF THE AIRPORT. THE  
GREATEST RISK OF SHOWERS MOVING NEAR THE AIRPORT WILL BE FROM  
CONVECTIVE ACTIVITY ALONG THE CHUGACH THIS AFTERNOON/EVENING.  
 

 
   
SHORT TERM FORECAST SOUTHCENTRAL ALASKA (DAYS 1 AND 2)
 
 
PERSISTENCE WILL BE A GREAT FORECAST FOR THE NEXT COUPLE DAYS.  
THE DRIVER WILL BE THE UPPER LEVEL LONG-WAVE TROUGH, BUT MORE  
IMPORTANTLY, THE COLD AIR CONTAINED WITHIN. THERE IS A WEAK SHORT-  
WAVE MOVING UP COOK INLET TONIGHT, WHICH LOOKS TO FOCUS CONVECTION  
ALONG THE WESTERN KENAI PENINSULA, KEEPING IT OUT OF THE  
ANCHORAGE BOWL. HOWEVER, IT WON'T BE FAR AWAY. THESE SHOWERS  
SHOULD BE MIXED WITH RAIN AND SNOW, POSSIBLY TURNING TO ALL SNOW  
IN THE SUSITNA VALLEY. INSTABILITY WILL BE INCREASING AS THE COLD  
CORE ALOFT MOVES OVERHEAD TOMORROW. SCATTERED RAIN SHOWERS WILL  
INCREASE IN COVERAGE DURING THE AFTERNOON, ESPECIALLY ALONG THE  
HIGHER TERRAIN OF THE CHUGACH. THESE SHOWERS SHOULD BE MOSTLY  
RAIN, BUT COULD SEE SOME ROGUE FLAKES OR EVEN GRAUPEL MIXING DOWN  
TO THE SURFACE IN THE MORE INTENSE SHOWERS. THE BACK EDGE OF A  
SHORT-WAVE WILL PASS LATE IN THE EVENING, BRINGING AN END TO THE  
SHOWERS, BUT THE BREAK WILL BE SHORT-LIVED.  
 
A MUCH STRONGER SHORTWAVE WILL BE DROPPING OUT OF THE BERING  
STRAIT REGION LATE THURSDAY EVENING. THE EFFECT WILL BE FIRST TO  
TIGHTEN THE PRESSURE GRADIENT, RETURNING STRONGER SOUTHEAST GAP  
WINDS TO TURNAGAIN AND KNIK ARM, SECOND TO RETURN RAIN AND SNOW TO  
MOST AREAS. THERE IS NOT ANY STRONG TEMPERATURES ADVECTION, EITHER  
WARM OR COLD ASSOCIATED WITH THIS SYSTEM, SO PRECIPITATION TYPE  
SHOULD BE STRONGLY TIED TO THE DIURNAL SURFACE/LOW-LEVEL  
TEMPERATURES. IT LIKELY WILL BE RAIN DURING THE DAY, MIXED WITH  
OR CHANGING TO SNOW AT NIGHT DEPENDING ON LOCATION/ELEVATION.  
 

 
   
SHORT TERM FORECAST SOUTHWEST ALASKA (DAYS 1 AND 2)
 
 
COOL AND WET WEATHER CONTINUE TO PLAGUE SOUTHWEST ALASKA THROUGH  
THE FORECAST PERIOD AS AN UPPER LEVEL LOW REMAINS IN PLACE OVER  
THE BERING, AND SHOWERY ONSHORE FLOW PERSISTS OVER THE AREA. A  
SERIES OF WAVES WILL ROTATE AROUND THE BASE OF THE UPPER LOW  
THROUGH FRIDAY, BRINGING ENHANCED SHOWER ACTIVITY TO THE BRISTOL  
BAY COAST AND UP THROUGH THE KUSKOKWIM RIVER VALLEY. MODELS ARE  
CURRENTLY INDICATING THESE ENHANCED SHOWERS TO LINE UP OVER THE  
BRISTOL BAY AREA OVERNIGHT TONIGHT INTO THURSDAY MORNING, WITH THE  
NEXT STRONGER WAVE TRACKING INTO THE SOUTHWEST MAINLAND THURSDAY  
NIGHT. THERE IS POTENTIAL FOR THESE SHOWERS TO BECOME MORE DEEPLY  
CONVECTIVE IN NATURE AS LIMITED SURFACE HEATING COMBINED WITH COLD  
AIR ALOFT WILL LEAD TO STEEP MID-LEVEL LAPSE RATES AND DECENT  
INSTABILITY THROUGH THIS EVENING. GUIDANCE CONTINUES TO SHOW A  
LINE OF ENHANCED SHOWERS FORMING FROM A LINE BETWEEN KING SALMON,  
ILIAMNA, AND SPARREVOHN THROUGH THIS EVENING. THUS, EMBEDDED  
ISOLATED THUNDERSTORMS MAY BE POSSIBLE IN THIS AREA.  
 
WITH THE SECONDARY SYSTEM TRACKING IN THURSDAY NIGHT, ANOTHER  
ROUND OF ENHANCED SHOWERS AND GUSTY WINDS ARE EXPECTED TO SPREAD  
FROM THE SOUTHWEST COAST INLAND THROUGH FRIDAY AFTERNOON. COLDER  
AIR ASSOCIATED WITH THIS SYSTEM AS WELL AS PLENTY OF ANTECEDENT  
COLD AIR AT THE SURFACE SHOULD KEEP PRECIPITATION AS SNOW FOR MOST  
LOCATIONS, THOUGH EXPECTED ACCUMULATIONS REMAIN MINIMAL AT THIS  
TIME.  
 

 
   
SHORT TERM FORECAST BERING SEA/ALEUTIANS (DAYS 1 AND 2)
 
 
A BROAD UPPER LOW OVER THE NORTHEAST BERING WILL REMAIN STATIONARY  
THROUGH THURSDAY MORNING BEFORE SHIFTING NORTHWARD OVER THE  
BERING STRAIT BY THURSDAY AFTERNOON. COOL AND SHOWERY CONDITIONS  
OVER THE EASTERN HALF OF THE BERING/ALEUTIANS ARE EXPECTED TO  
PERSIST THROUGH THURSDAY AFTERNOON, UNDER GUSTY WESTERLY FLOW.  
WHEREAS OVER THE WESTERN HALF OF THE AREA, SHORTWAVE RIDGING  
BEGINS TO BUILD OVER THE AREA TONIGHT, BRINGING DRIER CONDITIONS  
TO THE WESTERN/CENTRAL ALEUTIANS THROUGH THURSDAY AFTERNOON. WITH  
THE SHORTWAVE TROUGH DROPPING SOUTHWARD OVER THE EASTERN BERING  
THURSDAY, WE ARE EXPECTING COLDER TEMPERATURES AND SNOW SHOWERS  
TO PERSIST OVER THE AREA THROUGH THURSDAY NIGHT.  
 
BY THURSDAY EVENING THE NEXT SYSTEM ENTERS THE WESTERN  
BERING/ALEUTIANS, AND TRACKS EASTWARD THROUGH MUCH OF THE BERING  
AND ALEUTIANS BY FRIDAY NIGHT. THIS SYSTEM WILL SPREAD GALE FORCE  
WINDS AND RAIN ALONG MUCH OF THE CHAIN, AS IT PULLS WARMER PACIFIC  
AIR NORTHWARD AND TRACKS EASTWARD ACROSS THE ALEUTIAN CHAIN  
THROUGH FRIDAY NIGHT.  
 

 
   
LONG TERM FORECAST (DAYS 3 THROUGH 7)
 
 
A SYSTEM OVER SOUTHEAST ALASKA FRIDAY EVENING WILL QUICKLY MOVE  
EAST AND DISSIPATE FRIDAY NIGHT. A FAIRLY SIGNIFICANT UPPER LOW  
WILL BE OVER THE CENTRAL/EASTERN BERING FRIDAY NIGHT. THE LEADING  
FRONT WILL PUSH ACROSS SOUTHWEST ALASKA EARLY SATURDAY MORNING  
AND THEN INTO THE GULF AND SOUTHCENTRAL DURING THE DAY SATURDAY.  
THE UPPER LEVEL LOW WILL MOVE EASTWARD ACROSS THE GULF SATURDAY  
NIGHT. A FAIRLY STRONG SURFACE LOW WILL MOVE INTO THE BERING ON  
SATURDAY. AFTER THIS, THE MODEL GUIDANCE BEGINS TO DIVERGE. THE  
MODELS AGREE ON CONTINUED FAST ZONAL FLOW ALOFT, ALONG WITH THE  
GENERAL IDEA OF AN UPPER LOW IN THE BERING AND UPPER RIDGING OVER  
THE NORTHEAST PACIFIC INTO EASTERN ALASKA. HOWEVER, THE LOCATION  
AND TIMING OF VARIOUS SYSTEMS HAS A LARGE SPREAD IN THE GUIDANCE.  
THEREFORE THE FAIRLY ACTIVE WEATHER PATTERN WILL CONTINUE, WITH  
THE DETAILS NOT WELL DEFINED. THUS, THE WPC GUIDANCE BEGINS TO  
LEAN FAIRLY HEAVILY ON THE ENSEMBLE MEANS AFTER MONDAY.  
 

 
 
   
AFC WATCHES/WARNINGS/ADVISORIES
 
 
PUBLIC...NONE.  
MARINE...GALE WARNING 175 176 177 178.  
 

 
 

 
 
SYNOPSIS AND MODEL DISCUSSION...DEK  
SOUTHCENTRAL ALASKA...MTL  
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...TP  
LONG TERM...BL  
 
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