Northern Alaska Forecast Discussion National Weather Service Fairbanks AK 502 AM AKDT Wed Mar 29 2017
.SYNOPSIS... Aloft... A long wave trough over mainland Alaska will retrograde to the West Coast of Alaska and Eastern Bering Sea by Thu then remain there through the weekend into next week. The long wave ridge over NW Canada will build into Eastern Alaska on the weekend and persist into next week. This pattern change is setting up Northern Alaska for a much warmer and more active weather pattern as flow aloft turns southerly and increases from Thu into the weekend.
Several short wave troughs will drop from north of the Russian coast over the West Coast of Alaska and into the long wave trough over the next few days. The first trough will move south over the Chukchi Sea today and to the Yukon Delta Thu. There will be patchy clouds and flurries with this. The main affect will be north winds gusting to 25 mph with areas of blowing snow along the trough. A second trough will move to the northern Chukchi sea Thu pm and to the Western Arctic Coast to Bering Strait by Fri am. Will see more clouds and flurries with this feature...and north winds gusting 25 mph with areas of blowing snow.
A series of short wave troughs will make their way north over Interior Alaska over the next several days bringing snow or a chance of snow with each feature along with periods of southerly Chinook winds near the Alaska Range.
The first trough moving north is now over the eastern Brooks Range and will move north of the Arctic Coast this afternoon. Snow will taper off to flurries as it moves north of the Brooks Range.
The second trough moving north is in the Gulf of Alaska and will move north over the Alaska Range by 4am Thu...to Huslia to Old Crow by 4pm Thu...to Ambler to Barter island by 4am Fri and then elongating northeastward. This trough will feed moisture north over the Interior causing periods of light snow. Snow will be more focused along the surface frontal boundary that lies from Denali Park to Chandalar Lake. Will address snow below with the surface front. Snow will shut off just south of this trough as moderate chinook flows develops Thu. Will see south winds up to 40 mph along the Alaska Range Thu into Fri.
A third short wave moving north will move to the Alaska Range Thu night and over the Interior Fri. Expect downslope to prevent snow in the Eastern Interior Fri but there should be some snow in the Western Interior Fri.
Surface... A 1009 mb low just north of Deadhorse will move to 100 nm north of Deadhorse as a 1010 mb low by 4pm Wed...to 150 nm north of Deadhorse by 4am Thu...then dissipating. FLurries along this low will move offshore with the low.
An occluded front stretching from Denali to Chandalar Lake will remain stationary through Thu am. A low over the Kenai Peninsula will move to Denali Park as a 1008 mb low by 4am Thu...and to near Bettles as a 1008 mb low by 4pm Thu...and to just north of Barter Island as a 1006 mb low by 4am Fri. Snow along the front will increase today as the surface low feeds moisture north along the front. Expect 3-5 inches of snow along the front. Snow will shut off south of the surface low on thu. Will see much lighter amounts of snow with the low north of the Brooks Range.
.DISCUSSION... Models show similar solutions through Friday. See slight differences in precipitation location but GFS showed locations of snow best at 06z...so will use GFS locations. Start to see differences in short wave features Fri night into the weekend. Still agreement on the long wave pattern into next week. Will use a more ensemble approach for this weekend into next week the long wave trough remaining over Western Alaska with a ridge over the NW Canada and Eastern Alaska.
.AFG WATCHES/WARNINGS/ADVISORIES... Winter Weather Advisory for AKZ219-AKZ220-AKZ221.
Brisk Wind Advisory for PKZ210-PKZ220. &&
JB MAR 17
000 FXAK68 PAFC 291306 AFDAFC
Southcentral and Southwest Alaska Forecast Discussion National Weather Service Anchorage AK 506 AM AKDT Wed Mar 29 2017
.ANALYSIS AND UPPER LEVELS... An upper level closed low remains over the Yukon Delta region with an associated negatively tilted trough extending through the Bristol Bay interior into the northern Gulf this morning. This synoptic feature is reflected at the surface with a 998 mb low located over the western Prince William Sound region with an inverted trough extending northward towards the northern Alaska Range. The radar imagery is showing a large swath of echoes across the northern Gulf spreading inland into the Anchorage Bowl, and the Mat-Su Valleys. The surface temperatures are ranging from the lower 30s along the northern Gulf coastal communities, and in the upper 20s for inland areas.
The Eastern Aleutians has an amplified ridge extending into the central Bering with a weak closed upper level low located over the Western Aleutians. While south of the Central Aleutians a 968 mb storm force low is positioned at 44N 177W with an associated occluded front approaching the Chain. The satellite imagery is showing clear skies over the Southwest Alaska, and the Eastern Bering. The Western Aleutians/Bering has marine layer stratus. South of the Central Aleutians a storm force North Pacific low is intensifying this morning with an associated occluded front approaching the Central Aleutians.
.MODEL DISCUSSION... The models are coming into better agreement as the low pressure system in the northern Gulf tracks into the Prince William Sound region. The biggest forecast challenge remains the location of the heaviest snowfall for our AOR this morning. As the upper level trough swings through this evening, look for the snowfall to taper-off to a showery regime. Meanwhile, the models have a storm force low just south of the Central Aleutians with an eastward storm-track following the Chain. The preferred models of choice were the GFS/NAM, and the forecast confidence remains average.
.AVIATION... PANC...Look for steady snowfall with IFR to possible LIFR conditions through midday today before transitioning to prevailing MVFR conditions by this afternoon time-frame.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)... The negatively tilted upper trough will slowly push northeast today. This will force the Prince William Sound surface low inland as well, with snowfall moving inland and diminishing. There will then be a brief break tonight before the next front works its way into the gulf and south central Alaska from the southwest on Thursday. This next system is much stronger than the current one. It also has a lot more moisture, but most of the precipitation will fall in coastal areas as the strong flow down-slopes off the coastal mountains. The strong southerly fetch will also usher in warmer air, with temperatures at many locations approaching or even exceeding their normals for the first time this month.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)... Expect increasing clouds today with northerly flow across the Southwest and easterly flow developing along the Alaska Peninsula. A North Pacific low moves a front to the Peninsula late tonight bringing snow and strong gusty winds through Friday. Added areas of blowing snow to the forecast Thursday into Friday morning as the front elongates and stretches north to Bristol Bay. The low center approaches Sand Point Thursday afternoon, spreading snow to the Kuskokwim by Friday morning. Gusty easterly flow increases along the frontal boundary Thursday night which will bring a warm air surge across the Aleutian Range; therefore, precipitation will be mixed along and near the range. Temperatures warm across the Southwest on Friday as the front weakens and fills.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)... Ridging along the northern Bering shifts south to the western Aleutians by the end of the week as the strong North Pacific low moves over the eastern Bering waters and Southwest. The main impacts will be around the surface low as it moves near the eastern Aleutians late tonight with gale to storm force winds accompanied by rain and snow. Winds taper down late Friday as the surface low fills and lifts north. Cold air advects from the arctic to the Bering and the Aleutian chain into the weekend.
.LONG TERM FORECAST (Days 3 through 7)...
(From yesterdays Forecast Discussion)
Thursday will be the day that the upcoming pattern change will begin to be felt across much of Southern Alaska. An upper level trough is still expected to amplify in the coming days as strong jet support traverses with western periphery of the trough. This will help to advect and warmer and wetter air mass up towards Alaska. On Thursday, a surface low pressure south of Dutch Harbor will begin to weaken. However, the frontal boundary associated with this low and a surplus of upper level energy will help a triple point low to develop south of the Alaska Peninsula. This low is expected to move inland sometime late Friday morning into Friday afternoon but a a bit of model divergence has made it quite difficult to pinpoint all of the big details at this time.
With certainty, southern Alaska will see a pattern shift that will entail temperatures warming a good bit above normal and periods of rain and snow that will largely be confined to the northern Gulf Coast. As it appears now, Anchorage and other areas on the lee side of the Chugach Mountains will be downsloped from any precipitation with impressive cross barrier flow developing. Confidence is increasing that a chinook type pattern will set up over the Anchorage bowl on Thursday and/or Friday which could allow temperatures to warm into the lower to mid 40s. The temperature forecast is heavily dependent on the amount of wind that makes it into town but a warm up is nevertheless inevitable. A mix of snow and rain will likely be seen along the coast Thursday afternoon ahead of the frontal boundary but precipitation across most of Southcentral Alaska will quickly change to all rain as 850 temperatures warm to near zero degrees Celsius and sfc to 850 mb thicknesses rise quickly. Any precipitation from Friday morning onward across Southcentral Alaska will likely be rain except for portions of the Talkeetna Mountains, the Alaska Range and the Aleutian Range where some minor snow accumulations could be seen.
Heading into the weekend, chances for rainfall and possibly a mix of snow at times cannot be ruled out thanks to an active synoptic pattern. A series of lows will pass through the gulf of Alaska heading into next week but trying to come up with any low position is near impossible this far out as the models are quite volatile past 4 or 5 days. The weekend looks warm as temperatures will likely remain above normal and no reprieve is seen through the first half of next week.
SYNOPSIS AND MODEL DISCUSSION...PD SOUTHCENTRAL ALASKA...BL SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...KH LONG TERM...MC
000 FXAK67 PAJK 291405 AFDAJK
Southeast Alaska Forecast Discussion National Weather Service Juneau AK 605 AM AKDT Wed Mar 29 2017
.SHORT TERM...today through Wednesday night. South to Southwest flow aloft is the story over all of the AK Panhandle today. The embedded waves in this flow will bring periods of enhanced rainfall over the region in this wet pattern which will persist through Wednesday night.
The main change in the forecast stems from a wave developing just south of the Gulf of Alaska this afternoon which will move into the southeast Gulf tonight. This will not bring a big change in the winds but could extend the precipitation later into tonight across much of SE Alaska and could shorten the break between systems Thursday.
Current short term forecasts had some GFS guidance added. NAM also had a good handle on the developing wave which was more diffuse on the ECMWF. 06Z model runs of the GFS and NAM were continuing the development of this feature.
.LONG TERM.../Thursday through Tuesday night/ A high end gale force low is forecast to enter the western gulf on Thursday. Gale force winds with the associated front will arrive over outer coast on Thursday evening and extend from Cape Suckling south pas the Dixon Entrance. Peak conditions expected on the inside late Thursday night with gales forecast for Clarence Strait, Sumner Strait, southern portions of Chatham Strait, and Cross Sound. Elsewhere on the inside, small craft winds forecast except for northern Lynn Canal, where a parallel gradient should result in winds 20 kt or less.
Precipitation from this front will arrive along the outer coast late Thursday afternoon or Thursday evening. Significant low level warm air advection associated with this front will push snow levels higher, limiting snow to the higher elevations of the highways.
WPC, the 00Z GFS, and the 12Z ECMWF model runs presented moderate to good agreement at 500 mb on the placement of troughs, short waves, and upper level ridges. A ridge extending from BC southwest to coastal waters west of Washington State will become suppressed over the weekend but then rebuild early next week. Temperatures from Thursday into Friday were warmed slightly.
Confidence in the extended range forecast is average.
.AJK Watches/Warnings/Advisories... PUBLIC...None. MARINE...Small Craft Advisory for PKZ051-052. Small Craft Advisory due to Seas for PKZ022-041>043-053. &&