085 FXUS65 KPIH 221052 AFDPIH Area Forecast Discussion National Weather Service Pocatello ID 452 AM MDT Fri Mar 22 2019 ...FLOOD WATCH IN EFFECT FOR LARGE PORTIONS OF SOUTHEAST IDAHO THROUGH SUNDAY AFTERNOON... .SHORT TERM...Today Through Sunday Night. We continue to expect 2 rounds of precip to impact southeast Idaho over the next 3 days. The first will occur today and tonight as a low pressure system tracks north right across our forecast area. Snow levels are expected to average 6,500 feet, with 1-2 inches of snow possible above this level (slightly higher totals possible in the Bear River Range). Continued mild temps should keep even the higher-elevation highways predominately wet, and only isolated travel impacts are expected. Forecast confidence is moderate to high. A relative lull is possible from Sat morning into Sat afternoon, although we can't completely rule out a few isolated showers, and thus retain mention of such in the forecast region- wide. The second round of precip is forecast to arrive Sat eve in the central mntns and eastern Magic Valley, spreading east and continuing into Sun afternoon, as a trough of low pressure pushes through from the west. Snow levels will trend from near 6,500 feet Sat to 5,500 feet Sun. This second round will produce another 1-3 inches of snow in the mntns, with 4-5 inch totals possible above pass level in the central mntns. Slick travel will be possible over mntn passes across the region, but continued mild daytime temps may keep these in check. At this time, issuance of a Winter Weather Advisory seems unlikely given the forecast snow levels and totals, but we will continue to monitor trends. Drying and partial clearing from west to east is expected Sun night following passage of the trough axis. The greatest weather hazard we are monitoring during this entire period is the potential for flooding due to the combination of falling precip and mild temps. Please see the HYDROLOGY section below for more details on this potential, including anticipated QPF totals and possible impacts. - KSmith/Huston .LONG TERM...Monday through next Friday. The GFS and ECMWF remain in fair agreement throughout the forecast period but we continue to see run to run gyrations. The GFS holds the Pacific low further off the Oregon coast than it did just 24-hours ago and consequently is a little lighter on the precipitation now restricting any impacts to the CNTRL mountains Monday and Tuesday. The ECMWF continues to hold the low off the coast but is now painting some modest impulses of precipitation lifting NE through the CNTRL mountains Monday and Tuesday. By Wednesday, a fairly significant disturbance rotates through the base of the low and ejects inland across SRN Idaho Wednesday afternoon through Wednesday night followed by the remains of the Pacific low shearing east through the region Thursday. The GFS is pumping out some fairly impressive precipitation totals through this period while the ECMWF is more modest. The GFS favors a colder and drier airmass settling over the region from the north Thursday night into Friday whereas the ECMWF is a little slower with the drying. Have generally trended lower with the precipitation potential through this period but have not completely removed the thought of it. Elevated snow levels certainly support periods of rain in the valleys which should extend the rain-on-snow-melt issues through next week. - Huston && .AVIATION... Early morning satellite and radar imagery were showing a broad upper low over Utah with attending clouds and precipitation rotating WNW across the region. Models continue to support periods of valley rain and higher mountain snow with snow levels running upwards of 6500 ft MSL this afternoon, dropping to 5500 ft MSL overnight as the low shifts east and precipitation and cloud coverage begin to wane. Model time sections support MVFR CIGS at BYI, PIH, and IDA this afternoon and continuing through a good portion of the night while SUN and DIJ may stay predominantly VFR with brief periods of MVFR or IFR CIGS in showers. - Huston && .HYDROLOGY... The risk of flooding continues to increase across large portions of the forecast area. Flooding has been kept in check so far this week due to the nice slow/gradual nature of our warmup, temps dropping back solidly below freezing each night significantly slowing the melting/runoff, and a lack of any precip to exacerbate things. However, this will change today as a first round of rain and snow showers is rotating into the southern and eastern highlands and Bear Lake region this morning. This round of precip will continue to overspread much of the region today into tonight. Thus, we are transitioning from warm temps controlling our runoff, to a combination of slightly cooler but continued mild temps and active precip both contributing. Model guidance continues to hold steady with the higher QPF projections that appeared 24 hours ago. 0.25 to 0.50 inches of QPF are likely across the southern highlands, Bear River Range, and Bear Lake area through tonight, with generally 0.10 to 0.25 inches of QPF elsewhere across the region (perhaps slightly higher in the southern central mountains). Much of this will fall as rain below about 6,500 feet. Total runoff will, of course, include additional contributions from continued melting snow pack as well. Rain- falling-on-snow events, especially coupled with mild temps, are critical scenarios that can trigger increased flooding. The ground remains frozen across the region, meaning nearly all of the melting snow and precip will runoff vs. being able to soak in. Another round of precip is forecast Sat eve through Sun afternoon, producing another 0.10 to 0.20 inches of QPF region- wide with up to 0.50 inches in the central mntns west of Challis and Mackay and north of Hailey. Thus, the threat will continue through at least Sun afternoon before precip tapers off and low temps fall back below freezing Sun night. Forecast confidence in the flood threat and possible locations impacted has increased enough that a Flood Watch was issued yesterday afternoon across the southern Wood River Valley, Wood River Foothills, Shoshone Lava Beds, Arco/Mud Lake Deserts, Lost River Valleys, Upper Snake Plain, south central highlands, Raft River region, and Blackfoot Mountains through Sun afternoon. We expanded the Flood Watch this morning to also include the Beaverhead/Lemhi Highlands. The greatest flood risk remains in plain/valley areas where the warmest low-level temps overlap with snow pack available to melt and runoff coming out of the lower mntns, and the Flood Watch generally covers these areas of greatest concern. Consideration was also given to the QPF forecast (for example, there is less snow pack remaining to melt off below the forecasted rain-snow line across the southern highlands, but some impressive forecast QPF totals here suggest enough of a threat still exists to include the region in the watch). Ponding of water in low-lying areas, sheet and field flooding, and basement flooding are possible. Water may encroach onto some roads. Small creeks and streams may reach bankful and overflow, and rises are likely on larger rivers. At this time, we are not forecasting any of the larger rivers (such as the Big Wood or Big Lost) to reach flood stage, but trends in water levels will need to be closely monitored. Everyone should be alert for possible Flood Advisories or Flood Warnings over the next 3 days, and please report any observed flooding to our office when it is safe to do so as these help tremendously. - KSmith/Huston && .PIH WATCHES/WARNINGS/ADVISORIES... Flood Watch from noon MDT today through Sunday afternoon for IDZ051>053-067-068-075. Flood Watch through Sunday afternoon for IDZ057>059-062. && $$