Northern Alaska Forecast Discussion National Weather Service Fairbanks AK 240 PM AKDT Mon Apr 24 2017
.DISCUSSION... The current upper air pattern features a ridge that has persisted over northern Alaska the past few days. Currently this ridge sits along a Tanana to Northway axis. The ridge will begin to weaken tonight in response to a shortwave moving up along the west side of the ridge. This weakening will only be temporary as the ridge will build back along the same general orientation late in the workweek. A 510 dm low a 500 mb currently sits just east of Adak and will remain quasi-stationary until Thursday. At the surface, a low currently centered along 80 N north of Barter Island. A front stretches southwest from the low to Point lay. This front will move eastward tonight. A 1024 mb high sits in the Chukchi Sea north of the Chukotsk Peninsula.
North Slope: The past few mornings we have seen dense fog all along the Arctic Coast. With the aforementioned front moving east current thinking is that any dense fog tonight and tomorrow morning will be limited to areas from Deadhorse east. Suomi NPP VIIRS shows a large area of stratus behind the front, so as the front spreads eastward cloud cover will increase. There will also be some flurries tonight and tomorrow for areas behind the front, along with some areas of fog.
Central and Eastern Interior: It currently looks like today will be the warmest day of the week for most of the Interior. A cooling trend will begin tomorrow. A shortwave will move through the Interior tonight and tomorrow morning bringing along with it some scattered rain and/or snow showers. Any precipitation that does fall will be relatively light and for most places will only fall right as the shortwave is passing through. Cloud cover will also increase tonight across most of the Interior, with mostly cloudy skies persisting for much of the remainder of the workweek. Southerly winds with gusts of 35 to 40 mph are expected from late Tuesday morning into Tuesday evening in and near Alaska Range passes.
West Coast and Western Interior: A tight pressures gradient is bringing northeasterly winds with gusts of around 45 mph to St. Lawrence Island. This will persist through early Wednesday. Scattered rain and/or snow showers are expected Tuesday and Wednesday for much of the west coast and western interior south of the Bering Strait. Chances for showers will decrease on Thursday for the western interior but will remain for the west coast.
.FIRE WEATHER... Cloud cover will increase tonight across the Interior with mostly cloudy skies persisting for the remainder of the workweek for most locations. Monday will likely be the warmest day of the week for most Interior locations. Temperatures on Tuesday will be generally cooler with higher minimum relative humidity values. Southerly winds with gusts of 35 to 40 mph are expected from late Tuesday morning into Tuesday evening in and near Alaska Range passes.
.AFG WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory for PKZ210.
Brisk Wind Advisory for PKZ220-PKZ225. &&
612 FXAK68 PAFC 242352 AFDAFC
Southcentral and Southwest Alaska Forecast Discussion National Weather Service Anchorage AK 352 PM AKDT Mon Apr 24 2017
.ANALYSIS AND UPPER LEVELS... A persistent upper-level low controls the Bering Sea southward into the North Pacific. This is bringing in broad southeasterly flow over the Gulf of Alaska and most of the southern mainland.
Over Southcentral... Despite downslope, cross-mountain flow today in the Cook Inlet region... light rain dampened the dust in the lee of the mountains this morning with one trough travelling west-northwest. A second wave looks to repeat the process again tonight. Forecast challenges for Southcentral Alaska will be timing and setup of any rainfall across the region, especially in the lee of the Kenai and Chugach mountains.
Over Southwest Alaska and the Aleutians, the stacked low will continue the widespread circulation of clouds and showers. Larger forecast challenges may hold off until the end of the week when models indicate a building ridge over the Western Aleutians and a digging shortwave developing over the North Pacific.
.MODEL DISCUSSION... Models initialized well this morning but diverge with solutions over the Eastern Gulf of Alaska. The GFS resolves the surface features into a singular low pressure center. Forecasters prefer solutions keeping two circulations at this time. Therefore, the NAM is the preferred model today.
.AVIATION... PANC...VFR conditions will persist with light isolated showers continuing off and on through the night. Another wave of light rain will develop overnight and persist until early Tuesday morning. Challenges with timing and onset of any Turnagain Arm winds will persist through the evening. Previous forecasts have indicated arrival at the airport, however this is looking less likely as the day goes on.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)... The overall forecast hasn`t changed too much with the new forecast package but higher rain chances have been added to inland locations this evening as a short-wave moves across the Gulf of Alaska and Kenai Peninsula. Downsloping signatures have been quite weak as observed on the 12Z and 00Z RAOBS from PANC. Models really don`t develop much of a downslope signature until Tuesday morning following the passage of the shortwave which will help mitigate rain chances across interior portions of Southcentral Alaska Monday and Tuesday. The gulf coast will continue to see periods of light to moderate rainfall as the rainfall upslopes along the coastal mountains. Gusty winds have developed along Turnagain Arm and the Knik River Valley and will continue through Wednesday before winds really begin to diminish. The gusty winds through the Knik River Valley may decouple during the overnight hours at time which may lead for a reprieve in gusty winds but will quickly pick back up by the late morning hours on Tuesday. Cloudy skies will continue to be the norm through much of the week although some guidance is hinting at partially clearing skies later in the week.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)... Broad low pressure across the Bering remains stationary through Wednesday keeping flow generally out of the southeasterly direction. Weak disturbances rotating around this low brings continued chances for rain through mid week as moisture advects in from the Gulf. Patchy fog development late night will be a challenge across the Kuskokwim Valley and Delta as a weak boundary stalls through Wednesday.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)... A stacked low pressure system over the central Aleutians remains nearly stationary through mid week bringing lighter flow with higher chances for rain amongst the weak short waves rotating through the upper level flow. Temperatures cooling in the low and mid levels over the eastern Bering will create a steep temperature inversion just off the surface tonight and persist through early Wednesday. Areas of fog will develop across much of the eastern Bering under the inversion and shift around under the stagnant pattern.
.LONG TERM FORECAST (Days 3 through 7)... Southwest flow will persist through the weekend and will keep moderately unsettled weather across the region. Thursday and Friday, a low pressure system will slide north and into SE Alaska. In its wake will be an abundance of moisture and cloud cover across Southcentral AK. Friday and Saturday a closed Bering low becomes an open wave over the Southwest with a trough axis swinging over Southcentral which will bring increased chances for rain across the southern mainland.
Weak ridging will build in across the southern mainland into the weekend. Come Sunday, SE Alaska is expected to see a similar scenario pan out as what will be happening Weds/Thurs. Numerical guidance begins to struggle in day 5 and later time frame with run-to-run consistency issues and large ensemble spread across the Bering. There are similarities in timing of an elongated frontal/trough passage across the chain this weekend, but wildly different solutions on how strong and where the attached low will be.
.AFC WATCHES/WARNINGS/ADVISORIES... PUBLIC...NONE. MARINE...NONE. FIRE WEATHER...NONE.
SYNOPSIS AND MODEL DISCUSSION...DS SOUTHCENTRAL ALASKA...MC SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH LONG TERM...SS
000 FXAK67 PAJK 242304 AFDAJK
Southeast Alaska Forecast Discussion National Weather Service Juneau AK 304 PM AKDT Mon Apr 24 2017
.SYNOPSIS...A weather front across the northern gulf will lift north across northern Southeast Alaska tonight. A ridge of high pressure across the Panhandle will weaken and move east by Tuesday morning. A gale force low will approach the eastern gulf Tuesday.
.SHORT TERM...Weak lift in onshore flow has resulted in very light shower activity or sprinkles across parts of Southeast Alaska today, but rates and organization to rain bands appear to be increasing as vorticity advects northward into tonight. Still the main thrust of rain showers will be into the northeast and eastern gulf coast with only a chance of measurable precipitation spreading inland. Farther south, ridging behind the front will begin to suppress clouds with possible clearing into Tuesday morning. We have added some patchy fog mainly across the southern and central Panhandle late tonight early Tuesday morning. Confidence is higher for fog in the south.
Tuesday a developing system in the North Pacific will swing northeastward into the eastern gulf by late Tuesday. While agreement exists for a low, differences in track and position emerge as early as Tuesday afternoon which lowers confidence in some details of the forecast. A more easterly track toward Dixon Entrance like the ECMWF will slow rain`s arrival into the southern Panhandle. A westerly track would increase the rain`s efficiency of driving northward across the Panhandle. Thus, the chance of rain for the north may become bona fide light rain earlier.
We have kept the stronger winds across Prince of Wales Island but reduced them for many areas across the northern Panhandle as gradients will be small. Winds will increase along the coast, but we do not anticipate gusts above 30 miles per hour from Sitka northward late Tuesday.
From Wednesday into Wednesday night, model differences were smaller as all agree the low will remain in the eastern gulf, thus edits were more sparse during this time.
We used 12Z ECMWF/NAM for guidance especially from Monday night into early Wednesday. Temperatures were warmed a few degrees over the north Tuesday to account for ridging and sunnier skies ahead of the system approaching from the south.
.LONG TERM.../Wednesday night through Sunday/ The low will slowly lift north but will remain south of Sitka through Wednesday night before weakening and lifting east into the inner channels. Rain will remain a high threat south of the low but north it will be on the drier side. After the low moves into Canada there will be a period of onshore flow but towards the weekend it looks like the northern half of the forecast area could be dry with a low moving over Dixon Entrance. The southern areas will see a higher threat for precip over the weekend being closer to the low and near an area of deformation.
There is average forecaster confidence through the long term period. The overall trend in the models is to have bits of energy from the jet lift north into the gulf but overall most of the weather systems will be south of the area. For any changes done to the forecast the 00z nam, and 00z gfs was used through Thursday and then WPC after that.
.AVIATION...Predominately VFR conditions today with areas of lower clouds along the southern panhandle, western Baranof island as well as the northeast gulf into the Yakutat region of MVR for ceilings...although Yakutat has lowered to 800 feet this afternoon. Light winds up 15 kt for most of the areas...although a few higher winds and gusts near the showers as they track northward.
.MARINE...The front moving north across the northeast gulf tonight will briefly send winds to 25 kt in zone 52 tonight. We have kept the gale force easterlies in the southeastern gulf coast for the system approaching from the south late Tuesday as well as the small craft for Clarence Strait. Winds to the north were lightened through much of the period slightly.
.AJK Watches/Warnings/Advisories... PUBLIC...Strong Wind from Tuesday afternoon through Tuesday evening for AKZ027. MARINE...Gale Warning for PKZ041. Small Craft Advisory for PKZ036-042-052. Small Craft Advisory due to Seas for PKZ043.