Area Forecast Discussion

KEY MESSAGES

- Breezy winds and increasing risk for showers and thunderstorms Tuesday afternoon and evening into early Wednesday morning.

- Light to moderate rains Wednesday will result in the potential for small stream and urban flooding in SE WA.

- Wednesday also sees gusty winds and additional chances for thunderstorms.

SYNOPSIS

Unsettled weather returns Tuesday afternoon into Wednesday with potential for moderate rains, thunderstorms, and gusty winds. Main risks with thunderstorms will be outflow winds, small hail, and infrequent lightning. Moderate rainfall on Wednesday could result in small stream flooding in SE WA areas such as the Palouse. Gusty winds and additional chances for thunderstorms will also be seen on Wednesday.

DISCUSSION

Today through Thursday: The ridge that brought warming temperatures to the region is slowly moving eastward and out of the area as a low pressure off the west coast tracks inland. By midday today, active weather will return to the Inland Northwest as PoPs increase. Thunderstorms and moderate rainfall will be the two main risks with this incoming system. By Tuesday afternoon, the incoming trough combined with a frontal passage and warm air left in the wake of the departing ridge will create a chance for thunderstorms. CAPE values across the forecast area are between 200-400 J/kg. This potential will continue into Tuesday evening, and overnight into Wednesday. Thunderstorm chances will start in the Cascades midday Tuesday and then migrate eastward through the day, with southeast Washington and the Central Idaho Panhandle having the highest chances. Biggest impacts with these thunderstorms will be gusty outflow winds, small hail, and infrequent lightning.

As the low moves northeast and a deformation zone forms, much of the heavier precipitation will be confined to southeast Washington and the Idaho Panhandle. The majority of the forecast area will see at least a wetting rain (0.10 inches) by Wednesday evening. Wednesday morning through Thursday morning, much of the Idaho Panhandle and southeast Washington see 0.5-0.75 inches of rain. Isolated higher rainfall totals nearing 1-1.5 inches are possible in the WA/ID Palouse and in the foothills of the Central ID Panhandle. This will lead to rises on rivers and streams, and possible minor small stream and urban flooding in poor drainage areas. In particular, Kootenai River at Bonners Ferry is forecast to go into Action Stage by early Thursday morning. Stehekin River is seeing ongoing hydrologic issues, and is expected to reach Minor Flood Stage by later this morning. An important note, however, is that model spread with precipitation totals is very high. The spread between the 75th and 25th percentile precipitation totals for Wednesday is 0.5-0.8 inches. This is because of continued model disagreement in the exact location of the deformation zone. This is something we'll track closely over the next day.

With the frontal passage, winds will increase Wednesday afternoon, especially across the Cascades, Okanogan Valley and Highlands, Waterville Plateau, Moses Lake, Basin, and northern Blues areas, with gusts 20-25 mph expected and locally higher wind gusts near 35- 40 mph. Additionally, high temperatures will drop considerably after the frontal passage, nearly 15-20 degrees, from Tuesday to Wednesday. Low temperatures will not see quite as dramatic a drop, though it will still be noticeable at 10-15 degrees.

Friday through Sunday: As the low pressure moves into the northern central US and a trough strengthens off the coast of Canada, dry conditions will be seen through the weekend. The one exception will be showers in mountain areas due to orographic lift. Clusters are in disagreement about whether the low will retrograde westward as it interacts with another low off the California coast, so the main message right now is that there will overall be a warming and drying trend through the weekend. /AS

AVIATION

06Z TAFS: A closed upper-level low off the Oregon coast is pressing mid to upper level clouds into the Inland Northwest. Tuesday afternoon and evening, a system moves in from the south, bringing shower and thunderstorm chances to TAF sites. Main concerns with these thunderstorms will be gusty outflow winds, small hail, and infrequent lightning. There is a 20-30% chance of showers and thunderstorms for PUW, LWS, MWH, and EAT from 00-06Z Wednesday. Timing for showers and thunderstorms at GEG, SFF, and COE is the most uncertain part of this forecast, as many models show PoPs increasing to 20-30% right at 06Z. Bottom line, chances for thunder and showers increase right at the beginning of the next forecast period.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: High confidence for VFR conditions into tomorrow. Low to moderate confidence in showers and thunderstorm chances from 00-06Z Wednesday for PUW, LWS, MWH, and EAT. Low confidence in timing of PoPs for GEG, SFF, and COE as arrival rates are right along the 06Z forecast period boundary. Will keep a close eye on model runs for further timing. Highest impact alternate scenario would be storms and thunderstorms starting earlier for GEG, SFF, and COE, which would trigger an amendment. /AS

-----------------------

Confidence descriptors: Low - Less than a 30 percent chance Moderate - 30 to 70 percent chance High - Greater than a 70 percent chance

For additional probabilistic information for NWS Spokane airports,please refer to the Aviation Dashboard on our webpage: https:/www.weather.gov/otx/avndashboard

Preliminary Point Temps/PoPs

Spokane 75 47 56 39 57 35 / 0 60 80 30 30 10 Coeur d'Alene 76 46 52 37 54 33 / 0 60 90 50 50 20 Pullman 69 42 48 37 52 34 / 0 70 100 60 40 20 Lewiston 75 48 53 41 57 39 / 10 70 100 60 40 20 Colville 80 47 66 39 60 34 / 10 50 60 20 20 10 Sandpoint 75 47 54 38 52 33 / 10 50 90 60 60 30 Kellogg 77 46 51 38 49 33 / 10 50 100 80 70 30 Moses Lake 78 49 66 43 66 40 / 10 60 60 10 10 0 Wenatchee 76 52 63 46 65 43 / 20 50 40 10 10 0 Omak 79 53 69 42 65 39 / 10 40 20 10 0 0

OTX WATCHES / WARNINGS / ADVISORIES

WA...None. ID...None.