textproduct: Salt Lake City

KEY MESSAGES

- Gusty winds out of the southwest persist today across much of Utah and southwest Wyoming, with an emphasis on areas west of I-15.

- Tropical moisture advects north into Utah today increasing chances for precipitation.

- Threats for both flash flooding and isolated severe weather look to develop across central and southern Utah this afternoon with the arrival of tropical moisture.

- The active upper air pattern continues into next week (50-60% chance), potentially resulting in more precipitation by mid-week following an early week lull.

DISCUSSION

A large upper low continues to slowly move east through the Pacific NW, deepening with eastern extent. A tight southwest-northeast oriented pressure gradient remains in place, tightest across western UT resulting in another afternoon of enhanced southwesterly flow overhead. Most areas across UT and SW WY can once again expect to see gusts in the 25-35mph range, though areas west of I-15 may see gusts approach (and potentially exceed) 45mph given stronger winds aloft with higher surface temperatures to encourage stronger mixing.

This afternoon, moisture associated with Hurricane Marie will phase into the mean upper flow as as the southern stream nudges north and increases in magnitude. PWAT anomalies on the order of 0.5-0.8 above seasonal normals (potentially exceeding 200% normal values) are forecast to push north across much of Utah with this increase in southwesterly flow. Currently, SPC MesoAnalysis suggests that PWATs on the order of 0.8-1.2" of water have already begun to push into southern UT. Widespread ascent will strengthen as the southern stream phases into the mean flow with the aforementioned upper low, with a quick eastern progression of the main precipitation axis through the afternoon hours. As such, widespread showers and thunderstorms are forecast to continue developing across central and southern UT. Northern UT will also see precipitation this afternoon, though it is more likely due to frontogenetical forcing ahead of an incoming frontal boundary. Despite enhanced flow aloft suggesting fast moving storms, robust moisture content within the atmosphere continues to yield concern for flash flooding across central and southern UT, where a slight risk (level 2/4) for excessive rainfall has been outlined by the Weather Prediction Center (WPC). Rainfall totals have decreased across CAM guidance, suggesting lower rates with 0.3-0.5"/hr suggested on the higher end. Despite this, some instances of flash flooding remain expected given the anomalously moist airmass in place.

Regarding the severe threat, the forecast remains on track regarding an isolated threat across far NW UT. Ample shear exists on the order of 40-50kts ahead of the incoming frontal boundary, though only modest buoyancy develops in the midst of lower surface temperatures and drier air ahead of the front. The main hazards will be isolated gusts to around 58mph and instances of sub-severe hail less than 1".

Come Monday and Tuesday, the upper low progresses east leaving the forecast area in a dry westerly flow regime keeping conditions calm to start the week. Regarding a return to a wetter pattern, ensemble guidance has shifted the onset somewhat with roughly 50% of all ensembles supporting moisture return in some capacity to UT around the Thursday-Friday timeframe. Roughly 80% of all ensemble members support moisture return, and subsequently precipitation by Saturday- Sunday, though a notable shift has occurred within ensembles muddying the forecast once again. This will be monitored and the forecast will more than likely change as a result going forward given low confidence.

AVIATION

KSLC...VFR conditions will persist for the KSLC terminal through the period. Gusty southerly winds will continue during the day with clouds and moisture increasing this afternoon in the form of showers and some isolated thunderstorms. Winds will weaken overnight before a cold front moves through mid morning on Monday with a transition to northerly winds.

REST OF UTAH AND SOUTHWEST WYOMING...VFR conditions will persist for the entire airspace through the period. Stratiform rain will continue across southern and central portions of the airspace with embedded thunderstorms developing during the afternoon. Scattered thunderstorms develop late afternoon across the northern terminals. Winds remain strong out of the south before a cold front moves through the northern airspace mid morning on Monday.

FIRE WEATHER

Moisture will continue to gradually increase this afternoon as southwesterly flow pushes moisture further north. This afternoon, expect locally critical fire weather conditions primarily across the West Desert and Great Salt Lake Desert where humidities have been observed dropping below 15%. Additionally, strong wind gusts across areas primarily west of I-15 will range from 30-45mph each afternoon into Sunday.

Today, a stronger push of moisture arrives to Utah with widespread showers and thunderstorms across the state, though primarily across central and southern Utah. The highest rainfall totals are expected across higher terrain of central and southern Utah, though lower elevation areas further east may see higher isolated totals from stronger storms. For lower lying areas west of I-15, generally a trace or less of precipitation is expected today. Thunderstorms capable of wind gusts to around 50mph are possible across far northwest UT this afternoon, and a few scattered gusts to 58mph cannot be ruled out. A drying trend is expected to develop Monday and Tuesday with more precipitation expected after midweek next week across at least southern and central Utah.

SLC WATCHES/WARNINGS/ADVISORIES

UT...Flood Watch until midnight MDT tonight for UTZ113-114-116>120- 122-124>128.

WY...None.