Northern Alaska Forecast Discussion National Weather Service Fairbanks AK 136 PM AKDT Sat Mar 25 2017
.DISCUSSION... The 12z model suite initialized well against the 12 surface analysis and verified well against the 18z surface analysis. The 12z model suite continues to be in good agreement with previous model run output as well a displaying minimal spread between mode families in the short range as well as the mid range. Model spread increases significantly by the end of the week.
Aloft at 500 hpa, a longwave trough currently over eastern and central Alaska combined with a 500 hpa ridge currently over the eastern Bering Sea will slowly retrograde to the west through the upcoming week with the upper level trough moving over western Alaska and the upper level ridge moving over the western Bering Sea by late in the week. Weak southerly flow develops over eastern and central Alaska by mid week and bringing warmer temperatures to to areas that have had over three weeks of below normal temperatures. Temperatures are expected to reach freezing by around Wednesday or Thursday of next week for the first time in the month of March in Fairbanks. The warmer and moist south to southeast flow will bring the threat of light snow over the eastern and central interior by mid week as week overunning flow develops. Only minor snow accumulations are expected with this system.
Areas of dense fog continue to move in and out of locations along the the west coast under stable high pressure and will likely continue through tonight. Reduced visibilities in blowing snow will continue today along the north slope but will improve late tonight as winds diminish.
.AFG WATCHES/WARNINGS/ADVISORIES... None. &&
CCC MAR 17
000 FXAK68 PAFC 260115 AFDAFC
Southcentral and Southwest Alaska Forecast Discussion National Weather Service Anchorage AK 515 PM AKDT Sat Mar 25 2017
.ANALYSIS AND UPPER LEVELS... At the surface the dominant high pressure remains over the northern Bering Sea extending across interior Alaska. A strong low and the associated gale force front extends along the Aleutians to south of the Alaska Peninsula. An inverted trough extends from the low over the panhandle to the North Gulf Coast. Aloft we remain under the omega block as we have an upper low south of the western Aleutians with the upper ridge extending from the north Pacific across the western Alaska Peninsula into the Bering Sea and the upper trough that extends from the Arctic low across the mainland into the Gulf of Alaska.
The east flow along the North Gulf Coast is rotating showers along the coast. Off shore flow and dry conditions persist across south central Alaska to southwest Alaska. The stalled front along the Aleutians is holding the gale force winds across the region while keeping temperatures warm enough to keep the precipitation liquid west of Unalaska.
.MODEL DISCUSSION... The models remain in generally good agreement in the short term but continue to struggle with the placement of the low and westward extend of the precipitation Monday night and Tuesday. We have high confidence that snow will spread across Prince William Sound beginning Sunday night, however, the models have been limiting the westward extent of the precip area each successive run for the past couple of days, thus lowering our confidence in snow on the Cook Inlet side of the Kenai Peninsula.
.AVIATION... PANC...VFR conditions and light winds will persist.
.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2)... The Southcentral Alaska region remains under the influence of an upper level trough. This synoptic feature will gradually swing through the region, and is reflected at the surface with a weak low pressure system in the northern Gulf. Therefore, look for a showery regime with precipitation mainly along the northern Gulf coastal communities. By Monday the aforementioned low pressure system will have dissipated, but a 993 mb low in the lower southeastern Gulf will start to track to the northern Gulf. By Monday evening, the models have this system tracking onshore near Seward. This feature bring snow showers to the Eastern Kenai Peninsula, and the Prince William Sound region. Look for the Anchorage Bowl to have a chance for snowfall to spill- over the Chugach Mountains into the Bowl.
.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2)... Much of the same is in store for the Mainland for the next couple days. The only real change in the overall pattern comes on Monday morning when the Bering Sea blocking high retreats enough to allow the upper trough back over the area, bringing some slightly colder air with it. From a sensible weather perspective, not much will change. Dry and clear conditions will continue with generally light northerly winds. Expect some stronger winds and gusts during the daylight hours. Temperatures will remain below normal, especially at night as the clear sky promotes much cooler temperatures, mainly in protected valleys.
.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2)... A decaying frontal boundary will remain in place for the next two days. Light rain can be expected from Shemya through Atka as easterly winds begin to relax. A new low pressure system will bring a gale-force front south of the central Aleutians Sunday afternoon. Currently it doesn`t look like the front will spread over the chain. Otherwise general moderate easterly to northeasterly can be expected over the Bering Sea.
.LONG TERM FORECAST (Days 3 through 7)...
Little change to the longer range forecast for most of the region. By the day three timeframe, guidance is still in agreement that a surface low will spin into the Gulf as an upper level wave rides south from the interior. Coastal locations will see precipitation, but the question remains on the Anchorage area and how good of a chance there will be for snow in the area. As each day passes confidence increases and there is a decent chance that the Anchorage will see some snowfall into Tuesday.
At this time, confidence remains too low to go any higher than a chance of snow or to think about totals. The bottom line is that the pattern is changing. The longwave pattern undergoes a bit of a shift following this early week system and will set us up to be in a little bit of a favorable patter to see systems beyond the forecast timeframe reaching all of south central.
SYNOPSIS AND MODEL DISCUSSION...SA SOUTHCENTRAL ALASKA...PD SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...MTL LONG TERM...SS
000 FXAK67 PAJK 252317 AFDAJK
Southeast Alaska Forecast Discussion National Weather Service Juneau AK 317 PM AKDT Sat Mar 25 2017
.SHORT TERM...An upper trof will remain over the gulf through Sun. A developing low SW of Haida Gwaii will move N to the far SE gulf by late Sun afternoon. An occluded front will move N into the SE gulf and southern panhandle Sun.
Main forecast concerns will be precip potential/ptype/amounts, and winds. Scattered to locally numerous showers are over the eastern gulf and much of SE AK late this afternoon. The showers will decrease from the S this evening, with the far N area getting out of the showers by Sun morning as low level flow becomes more offshore ahead of approaching low and front. However, steadier precip with the front will move into the S late tonight then try to move further N Sun, but the offshore flow will tend to break up the precip area as it reaches the N half. E mtn slopes will be favored for precip over the S and central areas especially closer to the outer coast. The N should dry out Sun afternoon although there could be a few sprinkles around the Icy Strait area.
Ptype will be mainly diurnally driven. Warm boundary layer temps in the afternoon will generally mean rain or a mix, except in the heaviest showers where more snow may occur. As temps cool tonight, precip will become more snow especially over the N half of the area. Then on Sun, any snow should be mostly during the early morning before temps warm well above freezing. Snowfall will be highest around Yakutat where more frequent snow showers could lead to an inch or 2 of snow by late tonight. Elsewhere over the N, snowfall should be an inch or less, with most places getting little accumulation.
Winds will be increasing over the SE gulf later tonight and Sun as occluded front moves in. Expect strongest winds over the far SE gulf Sun afternoon where gales can be expected. Much of the remaining E gulf from Cross Sound southward will see winds pick up to 25-30 KT by Sun morning. Over the inner channels, winds will become more offshore and increase later tonight, with SCA level winds likely out of Cross sound and parts of Clarence Strait. Rest of the area should see winds pick up to the 15-20 KT range by late tonight and continue into Sun.
.LONG TERM...At upper levels southwesterly flow will last through mid week with a low extending down from the AK interior and ridging off the western CONUS. Surface low mentioned in the short term will continue its northwesterly track across the AK gulf with associated occluded front moving over the panhandle. The low will weaken by the time it reaches Prince William Sound Monday evening. Onshore flow will keep chance of precip over the panhandle, mainly for the northern half as ridging moves over the southern panhandle resulting in drier more spring like conditions. Next waves begin to move into the gulf mid week with first low tracking northeastward toward Prince William Sound followed by a more significant low and front moving over the gulf Thursday evening into Friday. Better chance of more significant rainfall with this later front. Began a trend to raise low temps due to more southerly flow later in the week which will likely begin a transition to more rain than snow.
Still fair amount of model spread for the Sunday night into Monday low. Used a blend of GFS/NAM with were in better alignment but did shift the low center further to the west and north. For later time periods ensembles were still in line with previous forecast so little change was made. Positive precipitable water anomaly and 850 mb temps with the late week system add to likelihood of a more significant rain event. Forecast confidence is average.
.AJK Watches/Warnings/Advisories... PUBLIC...None. MARINE...Gale Warning for PKZ041-042. Small Craft Advisory for PKZ021-022-036-043-051. &&