textproduct: Fairbanks

SYNOPSIS

Upper level ridging resulted in widespread sunshine today allowing for some of the warmest temperatures so far this summer, but clouds are approaching from the south associated with a low pressure over the Gulf of Alaska. This GoA low pressure approaches the Alaska Peninsula today where it stays nearly stationary through Monday before shifting east toward the panhandle on Tuesday. The primary weather maker for Northern Alaska over the next couple days is a secondary low pressure pivoting around the eastern side of the GoA low. This system tracks quickly northward across the Alaska Range on Sunday then to the eastern Arctic Coast on Monday. This system is tapping a moisture plume originating west of Hawaii so plenty of moisture will be available for clouds and rain chances. The highest chances and rainfall amounts are across the Brooks Range and North Slope Monday into Tuesday where rain and snowmelt results in rising water levels.

KEY WEATHER MESSAGES

Central and Eastern Interior...

- Red Flag Warning for Delta Junction and Isabel Pass Saturday and Sunday afternoons. South winds gusting up to 40 mph with RH values as low as 20% Saturday afternoon/evening. For Sunday aftn/evening, south winds gusting up to 60 mph and RH values as low as 30%.

- Red Flag Warning also for the Yukon Flats on Sunday with RH values as low as 20% and east winds up to 15 mph and gusts up to 25 mph.

- Breezy conditions are expected Sunday through Tuesday with easterly winds on Sunday and southwesterly winds on Monday and Tuesday as a storm system lifts north across the interior. This system also brings widespread clouds on Sunday followed by rain chances Sunday night into Monday in addition to cooler temperatures.

- The warmest temperatures of the summer so far with temperatures approaching 80F are occurring under mostly sunny skies Saturday afternoon and this shifts to the northern interior on Sunday.

- A few thunderstorms are also possible over the higher terrain of the southern interior through Saturday evening with gusty and erratic winds as high as 30 mph possible with any storms.

West Coast and Western Interior...

- Scattered thunderstorms are expected over the SW Interior roughly between Mountain Village and McGrath through this evening with gusty and erratic winds as high as 40 mph and small hail possible.

- Areas of low stratus and fog are expected to persist through the weekend across St. Lawrence Island and portions of the Norton Sound coastline.

- Highs mainly around 70F with the warmest interior valleys approaching 80F and the coolest coastal spots staying around 60F. Highs mainly in the 40s and 50s on St. Lawrence Island.

- A frontal boundary stalls roughly between Marshall and Ambler from Sunday through Tuesday focusing scattered showers and a few embedded afternoon thunderstorms and around 0.25" of rain with locally higher amounts possible in thunderstorms.

North Slope and Brooks Range...

- Seasonably mild on the Arctic Coast with highs in the 30s and 40s through the weekend. Warmer in the Plains and Brooks Range with highs in the 60s and even 70s.

- Moderate to locally heavy rainfall for the central and portions of the eastern Brooks Range and adjacent portions of the North Slope between Monday and Tuesday. Two day rainfall amounts of at least 0.5" are likely and a narrow band of heavier rainfall of 1" or more are possible. The exact placement of the heaviest rain is uncertain, but Anaktuvuk Pass north to Deadhorse is most likely that this time.

- The combination of snowmelt and rainfall is expected to result in rising river levels early next week, but rivers are mostly ice free so ice jams are not expected at this time.

FORECAST ANALYSIS AND DISCUSSION

Upper level ridging is draped across interior Alaska this afternoon to the north of a weakening vertically stacked low pressure that is approaching the Alaska Peninsula. The primary weather maker is a secondary area of low pressure pivoting around the aforementioned low pressure toward south-central Alaska. This low pressure system brings increasing cloud cover and embedded rain chances to northern Alaska Sunday afternoon through Tuesday afternoon in addition to gusty winds each day. As this system crosses the Brooks Range, it attempts to close off and intensify on Monday into Monday night resulting in a broad swath of moderate to locally heavy rainfall. Probabilistic guidance shows a broad area with >50% chances for at least 0.5" of rainfall from near Bettles to near Deadhorse with 40- 60% chances of more than an inch for the central Brooks Range by Wednesday morning.

A closer look at winds shows easterly winds becoming gusty Sunday afternoon across most of the interior ahead of the low pressure. These breezy conditions combine with a dry antecedent air mass to result in critical fire weather over the Yukon Flats and Isabel Pass/Delta Junction areas on Sunday. The cross barrier pressure gradient increases to between 4-6 mb Sunday afternoon and evening, which when combined with southerly flow aloft supports a 12 to perhaps 18 hour period of wind gusts up to 60 mph in Isabel pass. As the low pressure lifts north on Monday, gap winds decrease and south- southwest winds spread across much of the interior but more cloudiness and increasing moisture prevents RH values from reaching critical levels. On Tuesday winds turn or west-southwest and weaken as the low pressure moves over the Beaufort Sea.

Looking further west, the latest satellite imagery shows an increasingly agitated cumulus field over the Kuskokwim Mountains. ECMWF lightning guidance favors convective initiation in that area this afternoon before propagating westward toward the lower Yukon Valley through this evening. The frontal boundary that sparks these thunderstorms stalls roughly between Marshall and Ambler by Sunday afternoon where the next round of more isolated thunderstorms are expected. There appears to be too much cloud cover on Monday for renewed thunderstorm development and on Tuesday the front begins to push east into the interior resulting in more widespread rain chances. Increasingly broken cloud cover allows for more surface heating and destabilization as we get into midweek resulting in an upward trend in convective activity.

FIRE WEATHER

A breezy stretch of weather is expected from Sunday through Tuesday as a storm system lifts north from the Gulf of Alaska to the Beaufort Sea. Dry conditions ahead of this low pressure combine to result in locally critical fire weather conditions in the vicinity of Isabel Pass and across the Yukon Flats on Sunday. As of 1 PM Saturday afternoon, RH values have dipped to 27% at Ft Greely with values of 25-30% common across the upper Tanana Valley. Winds have so far underperformed, but southerly gap winds are expected to develop later this afternoon with gusts up to 40 mph possible south of Ft Greely. Additional heating should also allow RH values to dip a few more % before nocturnal recovery returns this evening. Looking ahead to Sunday, winds and cloud cover are expected to increase resulting in the opposite combination of wind/RH conditions near Isabel Pass. Gusts up to 60 mph in the pass and up to 50 mph near Delta Junction are expected Sunday afternoon and evening, but RH values may only dip into the 30-35% range due to increased cloud cover. Farther north, the Yukon Flats are still expected to have sunny skies resulting in their warmest day of the summer so far with temperatures approaching 80F and RH values dipping below 25%. Easterly winds accompany these hot/dry conditions with sustained winds around 15 mph and gusts up to 25 mph expected during the afternoon and evening.

A trend toward more widespread cloudiness sets in early next week resulting in cooler temperatures for the interior. Scattered thunderstorms are expected across the Yukon Delta Saturday afternoon and evening with isolated chances extending further north to the Brooks Range Sunday evening. While a few strikes can't be ruled out on Monday or Tuesday, abundant cloudiness should prevent surface heating needed for thunderstorm activity. Cooler temps with highs mainly in the 60s continue through the middle of the week. Breaks in the clouds become more frequent each day resulting in increasing thunderstorm chances and a warming temperature trend toward the end of next week. There is also an early signal emerging for southeasterly flow aloft supporting a considerably more active thunderstorm regime by next weekend.

HYDROLOGY

Most of the North Slope Rivers have broken up with little to no impacts. Moving forward as summer arrives and temperatures rise, there is a potential for snowmelt relative river stage rises. Temperatures stay mostly above freezing tonight and warm substantially on Sunday, perhaps above 60F in spots due to downsloping off the Brooks Range. Rain chances increase late on Sunday and may include a few embedded evening thunderstorms before transitioning to steady stratiform rain Sunday night into Monday night. Moderate rainfall amounts around 0.5" are likely across the central Brooks Range and adjacent North Slope foothills where most of the remaining snowpack is located. Locally higher amounts in excess of 1" are possible (~50% chance) between Sunday evening and Tuesday afternoon. This could lead to rapid melt and rising river levels due to the combination of rainfall and snowmelt. This contrasts with last year's late breakup when the snowmelt all entered the river systems while ice was mainly still in place.

Rising of water levels in rivers can be expected late this weekend or early next week.

EXTENDED FORECAST DAYS 4-7

Tuesday night through Friday night.

Weak upper-level troughing tracks south along the West Coast while ridging builds across much of mainland Alaska late this week supporting a gradual warming trend and a return to more seasonable norms. Precipitation chances are expected to decrease in coverage, but increase in intensity (i.e. convection) through the end of the week, particularly in the afternoon and evening hours. Confidence is fairly high in the overall ridge dominated pattern and southeast upper level flow on the southern edge of the ridge. However, smaller- scale disturbances embedded within the flow influence the day to day coverage and intensity of showers. Uncertainty remains with the exact timing, coverage, and location of any thunderstorm development, but confidence is increasing an increasing trend in thunderstorm activity is expected through the extended period.

Coastal Hazard Potential Days 3 and 4...None

AFG WATCHES/WARNINGS/ADVISORIES

AK...Red Flag Warning for AKZ933. Red Flag Warning for AKZ937. PK...Small Craft Advisory for PKZ801-850.