Area Forecast Discussion
Issued by NWS Grand Junction, CO
Issued by NWS Grand Junction, CO
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713 FXUS65 KGJT 151116 AFDGJT Area Forecast Discussion National Weather Service Grand Junction CO 516 AM MDT Sun Sep 15 2024 .KEY MESSAGES... - Showers and thunderstorms return to the area today as the leading elements of remnant moisture from TS Ileana moves into the region. - Conditions may become favorable for strong, organized thunderstorms Monday afternoon/evening which could result in localized wind gusts in excess of 60 mph and hail approaching an inch in diameter. - Above normal temperatures persist through Monday before a cold front moving through the region on Tuesday brings cooler than normal readings for the latter part of the week. - Aside from Wednesday, off and on showers can be expected across eastern Utah and western Colorado next week. && .SHORT TERM /THROUGH MONDAY/... Issued at 300 AM MDT Sun Sep 15 2024 A low amplitude trough approaching the region from the southwest is drawing moisture from the remnants of tropical storm Ileana that is currently over Arizona and New Mexico. The trough is expected to track over the forecast area today along with the moisture. The lift provided and moisture will support scattered showers through out the day. Low levels are fairly dry so cloud bases will be relatively high, which will limit rainfall amounts. The mountains have the best chance of measurable rain while valleys may only see sprinkles. Strong wind gusts could not be ruled out due to the high DCAPE. Subsidence behind the short wave trough combined with diurnal cooling will bring a downturn in shower/thunderstorm coverage this evening. Tonight a stronger trough moves southward over California and taps into even better remnant moisture currently in northern Mexico. The associated jet begins to nose over the region as well as diffluent flow aloft. Modest moisture advection (PWAT upwards of 200 percent of normal) together with the dynamics should support widespread showers and storms especially in the morning hours. The high-res models show a large swath of convection and stratiform that favors the southwest half of the area. The models continue convection into the afternoon as the low pressure begins to lift over Nevada. Wherever the clouds can break up CAPE could reach 500- 1000 j/kg, which appears like it may be the western half of the CWA. That in combination with shear supports the chance for stronger and more organized storms. These storms will be capable of gusty winds, large hail and heavy rainfall. Scattered convection looks to linger into the overnight hours as moisture advection does not let up. The highest rainfall amounts by late tomorrow evening are expected to be in the San Juans where upwards of an inch are possible (NBM probs of 50%). This poses a slight risk for localized flooding. There may also be a dusting of snow on the higher peaks across the forecast area. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... Issued at 300 AM MDT Sun Sep 15 2024 Showers and thunderstorms look to continue through the overnight hours Monday night as the deep low over the Great Basin lifts northeastward into Idaho by Tuesday afternoon. The cold front will be right on our doorstep moving across eastern Utah Tuesday morning and across western Colorado Tuesday afternoon. Moisture still remains to the east of the front with a much cooler and drier air mass advecting in behind the cold front. We will still see potential for some showers and storms, some of which could be strong, firing along the cold front and moisture gradient. Areas along and to the east of the cold front will be in the warm sector as well as the left exit region of a 100 kt jet streak. Gusty winds in the 30s and 40s remain possible as this trough axis and cold front swing through Tuesday afternoon. PWAT anomalies will drop from 120 to 150 percent of normal ahead of the front to 50 to 80 percent of normal behind the front. So, safe to say, this cold front will sweep out whatever moisture is left. Temperatures drop to around 5 to 10 or so degrees below normal for highs on Tuesday. Clearing skies Tuesday night along with the drier air mass post-frontal will allow for efficient radiational cooling and our lows quite cool and fall-like. On Wednesday, we will be in a transition period with much drier conditions as one low moves out and another dives into the Great Basin. There is quite a bit of model variance beyond Wednesday with how to handle this next system. The GFS brings this low quickly across our CWA as an open wave trough on Thursday, while the ECMWF keeps this trough over the Great Basin Thursday before bringing it across our CWA on Friday. One thing the models seem to agree on is that this system seems to be lacking in the moisture department unlike the previous system. Despite timing differences, this system also helps reinforce the cooler air as highs in the wake of this system will drop from around 5 degrees below normal to between 5 and 10 degrees below normal. Snow is possible on the higher peaks in the wake of this system due to the cooler air mass. The question really is when this occurs. The blended guidance maintains chance PoPs with unsettled conditions Thursday through Saturday with the best potential Thursday night into Friday...kind of a consensus between the models. Northwest flow takes hold behind this system as a ridge builds just off the West Coast, keeping cooler below normal temperatures in place. && .AVIATION /12Z TAFS THROUGH 12Z MONDAY/... Issued at 513 AM MDT Sun Sep 15 2024 Showers and storms are possible over the higher terrain this afternoon and some may drift over a few sites. Gusty outflow winds around 40 mph will be the main concern. Additional shower development is possible late tonight at some of the sites. VFR conditions are expected to prevail over the next 24 hours. && .GJT WATCHES/WARNINGS/ADVISORIES... CO...None. UT...None. && $$ SHORT TERM...KJS LONG TERM...MDA AVIATION...TGJT