Area Forecast Discussion
Issued by NWS Pocatello, ID
Issued by NWS Pocatello, ID
496 FXUS65 KPIH 062049 AFDPIH Area Forecast Discussion National Weather Service Pocatello ID 249 PM MDT Mon May 6 2024 .SHORT TERM...Tonight through Tuesday night. Active northwest flow pattern continues with series of shortwave troughs moving through the mean trough over the northern Rockies. Satellite imagery shows first shortwave shifting east of the region this afternoon, with weak ridging ahead of next negatively aligned shortwave. Strong upper jet just south of the region helping to amplify winds across the southern half of East Idaho, while still providing for mainly orographically induced showers over higher terrain. With the transition mainly to more convective influence, both QPF and subsequent snow totals through the short term tail off significantly with last night`s system shifting out of the region. Have dropped the remainder of the Winter Weather Advisories and Winter Storm Warnings for today. Additional snowfall is expected Tuesday into Tuesday night over mainly higher elevations, but even at the 90th percentile (1 in 10 chance of exceedance) higher end snowfall totals with potential travel impacts stay at or below 6" accumulation. Winds and temperatures remain the far greater story for impacts. Very strong winds continue mainly across the southern portions of the Snake Plain, southwest of US-26. Observations show widespread gusts over 45 mph, which should continue into the evening with support from strong mid-level flow, steepened surface gradient, and enough sunshine to mix to the stronger winds aloft. Have dropped the Wind Advisory for the Arco Desert/Mud Lake zone as most of the northwest winds across the region are struggling to reach minimum thresholds. Elsewhere, expanded the Wind Advisory into Tuesday evening, with the caveat that the winds should briefly subside this evening below minimums, but still windy at around 20 mph sustained. Guidance suggests that the winds will increase well before sunrise, generally in the 2-4am timeframe, and could be stronger than today. The strongest winds Tuesday should remain from the Eastern Magic Valley into the Raft River region and Lower Snake Plain. 90th percentile NBM favors widespread region with gusts over 50 mph, possibly exceeding 60 mph in the typically overperforming Raft River region and I-84 south of the I-86 interchange. A High Wind Warning may be needed in that area if the winds overperform from todays readings. Lastly, temperatures tonight and Tuesday night will continue to be well below normal. A Frost Advisory has been issued for the entirety of the Snake Plain growing zones, with some potential to drop below freezing in outlying areas that see a more significant drop off in winds overnight. A Hard Freeze is not anticipated tonight. Similar temperature profiles are expected Tuesday night, with less wind, but the NBM ensemble probabilities do indicate a potential trend of a degree or two cooler Tuesday night. This could tip the scales toward a Freeze Warning with localized potential for a Hard Freeze. DMH .LONG TERM...Wednesday through next Monday. Seasonably cold and unsettled weather will continue into Wednesday as a H5 low situated over the NRN Great Plains brings colder, northerly flow overhead and isolated to scattered light rain/snow showers throughout the day further north and east. While heavier rain/snow will exist north and east of the Continental Divide, this flow regime will keep our area mostly shadowed with lesser amounts. As that H5 low over the Plains ultimately splits Wednesday night into Thursday, a secondary H5 low will begin to work SW through the Great Basin. This will keep very isolated precipitation chances going south and east of the Snake River Plain but what will be more noticeable will be the warmer temperatures. Highs in the 40s/50s on Wednesday will increase to the 50s/60s on Thursday as a ridge of high pressure begins to shift inland over the PacNW and NRN Rockies. High confidence amongst ensemble, deterministic, and cluster models shows a broad, positively tilted H5 ridge building in from the Pacific to round out this week. What this means will be the warmest airmass of the season lifting NE into our area as predominant SW flow ushers in highs back in the 60s/70s with even some low 80s across our lowest elevations as soon as Saturday. Our current forecast continues to represent the NBM 50th percentile high temperatures well in this regard which remains in line closest to the EPS. While warm and dry conditions are expected Thursday through Monday, as that H5 jet overhead becomes more zonal by early next week, we may begin to see some moisture working overtop this ridge feature helping to re-introduce some slight chances for precipitation. The GEFS and EPS both support this solution helping to bring in some rain shower/thunderstorm potential as we head into next week. Temperatures however look to remain relatively unaffected as highs in the 60s/70s prevail. MacKay && .AVIATION...For the 18Z TAFS through 18Z Tuesday. A mix of VFR/MVFR conditions continue this afternoon with a mix of rain/snow showers around KIDA and KDIJ as strong winds persist across the Snake River Plain and Magic Valley. Winds are slightly less at KSUN and KDIJ but still expect gusts to around 20-25 kts. As a shortwave trough begins to build in from the west this evening through the overnight hours, isolated precipitation chances will continue into Tuesday with MVFR/IFR CIGS/VIS possible at times associated with heavier bands of rain/snow. Strong winds will persist into Tuesday of which will be strongest at KPIH and KBYI around 25-35 kts with gusts to 30-45 kts. Marginal instability this afternoon and evening will support a nonzero chance of thunderstorms around KBYI and KSUN but with the HREF model probability of thunder now staying less than 20%, have kept the mention out of thunder out of the TAFs at this time. MacKay && .HYDROLOGY...Area rivers remain elevated today, with the Portneuf and Blackfoot Rivers remaining the trouble spots of the day with Flood Warnings continuing. Cool temperatures and ongoing periods of showers (snow at higher elevations) will continue to slightly mitigate snowmelt runoff. Periods of showers will continue to provide additional water into the basins, mainly snow at mid slope and higher elevations. Blackfoot River levels look to stay at Minor Flood for the foreseeable future. Portneuf forecast trends continue to produce a temporary nudge back into the low- end Moderate Flood category, then drop back into persistent Minor Flood for several days. The cold temperatures and consistent unsettled conditions should keep East Idaho waterways high, fast and cold through the week. DMH && .PIH WATCHES/WARNINGS/ADVISORIES... Wind Advisory until 8 PM MDT Tuesday for IDZ051-054>057. Frost Advisory from 8 PM this evening to 9 AM MDT Tuesday for IDZ051>055. && $$