I wasn't expecting to return to the topic of summer rain quite so soon, but for two reasons it makes sense to do so. First, the current weather forecast from NWS Fairbanks is worthy of comment, with a flood watch in effect over the weekend for the central interior, middle Tanana valley, and Denali region. The rainfall forecast for the next 3 days is striking to say the least; any time you have 15% or more of your normal annual precipitation predicted for a weekend, it's worth paying attention.
The special statement issued by the NWS includes quite a collection of hazards:
SPECIAL WEATHER STATEMENT NATIONAL WEATHER SERVICE FAIRBANKS AK 508 AM AKDT FRI JUL 31 2020 AKZ220>226-011415- YUKON FLATS AND SURROUNDING UPLANDS-CENTRAL INTERIOR- MIDDLE TANANA VALLEY-DELTANA AND TANANA FLATS- UPPER TANANA VALLEY AND THE FORTYMILE COUNTRY-DENALI- EASTERN ALASKA RANGE- INCLUDING FORT YUKON, VENETIE, CENTRAL, CIRCLE, STEVENS VILLAGE, BEAVER, CHALKYITSIK, BIRCH CREEK, CIRCLE HOT SPRINGS, EAGLE SUMMIT, TWELVEMILE SUMMIT, NENANA, ANDERSON, TANANA, MINTO, MANLEY HOT SPRINGS, RAMPART, LAKE MINCHUMINA, LIVENGOOD, FAIRBANKS, FORT WAINWRIGHT, EIELSON AFB, ESTER, NORTH POLE, MOOSE CREEK, TWO RIVERS, FOX, CHATANIKA, CHENA HOT SPRINGS, SOURDOUGH CAMP, SALCHA, DELTA JUNCTION, FORT GREELY, HARDING/BIRCH LAKE, DRY CREEK, DOT LAKE, HEALY LAKE, TOK, TANACROSS, EAGLE, TETLIN, NORTHWAY, ALCAN, CHICKEN, BOUNDARY, HEALY, DENALI NATIONAL PARK, CARLO CREEK, KANTISHNA, MENTASTA LAKE, BLACK RAPIDS, DONNELLY DOME, TRIMS DOT CAMP, EAGLE TRAIL, AND MINERAL LAKE 508 AM AKDT FRI JUL 31 2020 ...COPIOUS RAINFALL LIKELY OVER MOST OF THE EASTERN/CENTRAL INTERIOR AND ALASKA RANGE SATURDAY EVENING... ...INCREASING LIKELIHOOD FOR A SIGNIFICANT OUTBREAK OF STRONG THUNDERSTORMS OVER THE EASTERN INTERIOR... STRONG THUNDERSTORMS ARE EXPECTED BEGINNING LATE SATURDAY AFTERNOON OVER THE FORTYMILE UPLANDS AND WHITE MOUNTAINS. THESE STORMS WILL LIKELY PRODUCE DANGEROUS LIGHTNING, LOCALLY TORRENTIAL RAINFALL RATES, HAIL, AND STRONG WINDS. EXPECT CONDITIONS TO DETERIORATE RAPIDLY FROM THE HOT AND MOSTLY SUNNY AFTERNOON CONDITIONS. OUTDOOR RECREATORS, IN PARTICULAR, SHOULD EXERCISE SIGNIFICANT CAUTION AND BE WEATHER-READY. EXPECT TORRENTIAL RAINFALL RATES WITH UP TO AN INCH PER HOUR OVER LOCALIZED AREAS. TOTAL RAINFALL WILL RANGE FROM 0.5-2" TOTAL OVER THE EASTERN INTERIOR UP TO 2-4" OVER THE CENTRAL INTERIOR, WITH THE HEAVIEST TOTALS CURRENTLY EXPECTED FROM TANANA TO MINCHUMINA TO THE MINTO FLATS. HEAVY RAIN WILL TRANSITION TO LIGHTER RAIN, BUT CONTINUE THROUGH THE WEEKEND OVER THE CENTRAL INTERIOR. RIVER LEVELS REMAIN ELEVATED WITH MUCH OF THE SURROUNDING TERRAIN STILL SATURATED FROM WEEKS OF CONTINUOUS WET WEATHER. THIS MEANS THERE IS POTENTIAL FOR SHARP RISES ON RIVERS INCLUDING THE CHATANIKA, CHENA, LITTLE CHENA, SALCHA, AND GOODPASTER, AS WELL AS SMALLER CREEKS AND TRIBUTARIES ACROSS THE EASTERN INTERIOR. LARGER RIVERS INCLUDING THE TANANA RIVER WILL BEGIN RISING EARLY NEXT WEEK AS WATER MOVES THROUGH THE SYSTEM. MUDSLIDES AND QUICK RISING STREAMS IN THE ALASKA RANGE, ESPECIALLY NEAR DENALI, WILL ALSO BE A SIGNIFICANT CONCERN. FOR THE LATEST RIVER INFORMATION GO TO WWW.WEATHER.GOV/APRFC. FOR THE LATEST UPDATES VISIT WEATHER.GOV/FAIRBANKS
The weather pattern responsible for all this involves a strong upper-level trough over the Gulf of Alaska, with warm and moist flow reaching the interior from the southeast. This bears some resemblance to the rainy weather setup in late June. The precise origin of the forcing for this weekend's expected deluge is not quite clear at first glance, but there's no doubt about the magnitude of the rainfall signal in the model forecast - see below.
This latest onslaught of moisture will add to the year's precipitation surplus and will favor increased permafrost thawing this summer. That's according to the conclusions of a new paper published by Tom Douglas of the Army Cold Regions Lab in Fairbanks:
Increased rainfall stimulates permafrost thaw across a variety of Interior Alaskan boreal ecosystems
This work was highlighted by Ned Rozell back in June:
The paper shows that permafrost active layer (seasonal thaw layer) depth increased significantly across a diverse set of locations near Fairbanks from 2013-2017, and there seems to be a correlation between the annual thaw depth and summer rainfall totals, with wetter summers tending to produce more thaw. This makes sense, as surface water above 10°C carries a substantial amount of sensible heat as it percolates down to the 0°C top of the permafrost; and the effect may be multiplied in areas where thawing permafrost is subsiding, leading to greater liquid water influx from surrounding areas.
One slight surprise with the results (as discussed in the paper) is that the dramatic thaw in the record-wet summer 2014 was not reversed in 2015 or 2017, despite those summers being less wet. But curiously, the rainfall data in the paper seems to be slightly off: the stated rainfall totals for 2015 and 2017 are quite a bit lower than what actually occurred in Fairbanks, so the "problem" is not as great as the discussion suggests. As a recent post here showed, every year since 2014 has been considerably wetter than normal in Fairbanks (with the vast majority of the increase coming as rain), so it's no surprise that permafrost hasn't had a chance to recover. And from the point of view of rainfall, it certainly isn't doing any better this year.
Update Saturday 5pm:
Here's a satellite loop showing the frontal zone developing over the southeastern interior. Progress is a bit delayed, but big rain is coming.
Today's GFS forecast:
This afternoon's NWS forecast discussion is worth saving for future reference:
Northern Alaska Forecast Discussion National Weather Service Fairbanks AK 222 PM AKDT Sat Aug 1 2020 .SYNOPSIS... A significant storm system is expected to impact the Southern Interior and Alaska Range beginning late this afternoon and continue through late Sunday or Monday morning. A strong front is expected to stall along a Fairbanks to Denali National Park line and will produce very heavy rain, perhaps record setting rainfall in the Alaska Range and Southern Interior. The front has slowed down and will not arrive during peak heating this afternoon so the chances for strong thunderstorms in the Central Interior have diminished some, however chances for strong thunderstorms remains high in the Eastern Interior this afternoon and evening. Very heavy rain is expected through monday Morning including the Tanana Valley where 1 to 3 inches of rain is expected through Monday morning and in the Alaska Range especially near Denali National park where 2 to 4 inches of rain is expected with local amounts pushing nearly 6 inches. The potential for local flash flooding is a very high across much of the Alaska Range, especially around the Denali area. Elsewhere, cooler and showery conditions will persist over the West Coast and Western Interior with continued fog and stratus over the North Slope. && .DISCUSSION... Upper Levels and Analysis... A strong ridge over the Central and Eastern Interior will weaken and retreat to the north as strong upper level low over the panhandle pushes northwest into the southeast interior this evening and continues to push northwest into the Central interior near Fairbanks tonight. 850 mb temperatures currently around 14 above will drop rapidly to around 6 above by Sunday afternoon. Rapid cooling aloft combined with diluent flow aloft will produce a closed low centered near Fairbanks late tonight and Sunday. This closed low will produce northerly flow at the surface and aloft over the Central Alaska Range and will enhance rainfall with moderate upslope flow. The low stretches along an upper level boundary along a line from Fort Yukon to Fairbanks to Denali National Park. Very heavy rainfall is expected to fall along this stalled front tonight through Monday morning. Over the North Slope, southwest flow and a stationary front remain in place, keeping showers and clouds and cooler weather there, as well. Model Discussion... The models continue to struggle badly on the timing and placement and amount of heavy rainfall over the Central Interior and Central Alaska Range even at less than 6 to 12 hours before the start of the event. We utilized a blend of models again with a heavier weighting of the ECMWF due to its consistency over the last several runs on the timing, placement and amount of heavy
rainfall and retained reasonable continuity with previous forecast packages and latest model updates. The heavier weighted ECMWF forecast favors widespread heavy rainfall from the Alaska Range to the Tanana Valley, thus ongoing flood watches will remain in place. Very small differences in feature location will result in significant differences in timing, placement and amounts of heavy rainfall especially with the upslope precip enhancing and downslope precip limiting terrain of the Alaska Range with very subtle changes in wind direction. Central and Eastern Interior... A very nice afternoon with sunny and warm conditions ongoing over the Central and Eastern Interior as the upper level ridging continues to build and temps rise into the 80s. We will likely make a run at the warmest day of the summer this afternoon before a strong front pushes in from the east. The sunny, nice weather will deteriorate rapidly beginning late afternoon over the Eastern Interior and this evening over the Central Interior as a strong front pushes into the area from the southeast. Strong thunderstorms are expected to develop over the Fortymile Uplands, Eastern Alaska Range and White Mountains then move westward late this afternoon and evening. Instability will be very high as CAPE values will exceed 1500-1800 j/kg, a high value in Alaska for instability. Storms will produce may produce torrential rainfall in excess of an inch per hour in some areas, along with hail, deadly and frequent lightning, and strong winds. By late evening and overnight, the storms will merge into a complex and shift west over the Tanana Valley and Western Alaska Range northward into the Central Interior. There remains some uncertainty on the exact timing, location and amounts of the heavy
rainfall but at this point we expect heavy rain rain over the entirety of the Alaska Range with moderate to heavy rainfall over the Tanana Valley by late tonight and Sunday morning. Expect rainfall to continue through Monday morning, at least, with the potential for 1-3" of rain in the Tanana Valley including Fairbanks and 2-4" across the Alaska Range, highest values near Denali possibly reaching 6 inches through Monday morning. Thunderstorms and moderate rainfall will also impact areas north including the southeast Brooks Range and Yukon Flats, although flooding rainfall is not expected. West Coast and Western Interior... Quiet weather will persist out west, especially along the coast as a northerly wind gradient at the surface increases. Some fog and
stratus will remain a problem over the immediate coasts, however, Interior will gradually warm up with threats for isolated thunderstorms each afternoon through Monday. North Slope and Brooks Range... A stalled front continues to sit over the North Slope, bringing light rain and continued clouds. This will weaken, and southerly flow will increase over the eastern half of the Brooks Range and North Slope tonight and Sunday, bringing rain and possibly an isolated thunderstorm over the southern slopes of the Brooks Range. This will last through Monday while the remainder of the North Slope will continue to see cooler and stratus/fog conditions given the light onshore northeast flow. Coastal Hazard Potential Days 3 and 4...None. && .FIRE WEATHER... One final hot day is in store for the Central/Eastern Interior this afternoon before a powerful storm rolls through this evening and overnight. There will likely be a lightning outbreak over the Eastern Interior with widespread lightning, but storms will also bring copious rainfall. Extensive thunderstorms will also impact the drier areas of the Interior including the Yukon Flats. Expect heavy rainfall from the Alaska Range to the Central Interior and Tanana Valley through Monday. Cool abut mostly dry with isolated
thunderstorms will be the main story over the Western Interior. The West Coast will remain cool and dry. && .HYDROLOGY... The focus continues to be an incoming heavy near record setting rain event for parts of the Central and Eastern Interior beginning late this afternoon and into Monday. Models continue to struggle with the placement of the heaviest precipitation in the Interior (see model discussion above), but the emerging concensus is a broad area from Delta Junction to Fairbanks to Lake Minchumina southward into the Alaska Range with 1 to 3 inches of rainfall; the highest bullseye of rainfall of 2 to 4 inches with 6 inches locally remains the Alaska Range near Denali National Park through Sunday. Flood Watches remain in effect for these areas. Expect sharp rises on smaller rivers including the Chena, Little Chena, Salcha, Nenana, Delta, and Goodpaster Rivers with continued rises along the Tanana from Fairbanks west. Rock and mud slides are possible in the steep terrain of the Alaska Range along with the possibility of local flash flooding in torrential rainfall.