119 FXUS65 KSLC 310030 AFDSLC Area Forecast Discussion National Weather Service Salt Lake City UT 630 PM MDT Tue Jul 30 2019 .SYNOPSIS... A surge of monsoon moisture will bring locally heavy showers and thunderstorms Wednesday and Thursday. A somewhat drier southwesterly flow will develop for the end of the week. && .DISCUSSION... Experimental CIRA Advective Precipitable Water Product shows a substantial increase in moisture during the past 24 hours along and south of I-70, primarily in the 700-500mb layer, but extending down to the surface as well. Surface dewpoints currently in the 40s/50s most valleys except West Desert, which are expected to climb well into the 50s tomorrow. Lightning has trended lower early this evening as a lull of instability is taking place. Updated the forecast to account for recent radar trends, increasing PoPs across southwest Utah this evening. Have extended low end PoPs northward to just south of I-80 for the second half of the night. HREF indicates as moisture continues to deepen overnight, instability increases and advances northward. While QPF especially further north will be light, if any, there will be a threat of locally gusty microburst winds as the lower levels continue to saturate. The majority of Convective Allowing Models show these trends in their Simulated Reflectivity and Wind Gust output. The combination of a developing low level jet, increasing clouds and moisture, and today's hot temperatures will lead to a very warm night along the Wasatch Front. Some locations may not dip below 80F. Models indicate several weak MCVs congeal into a more focused feature overnight which translates northeast across northern Utah tomorrow afternoon. Given the rich moisture profile and better instability, thickening Warm Cloud Layer and dynamical forcing, locally heavy rainfall is a concern. Will be watching for the potential for Flash Flooding. However, the MCV will introduce increased cloud cover, and its track/timing will dictate areas of best lift and stronger subsidence in its wake. Therefore there is higher than typical uncertainty as to the Flash Flood threat timing and location. && .PREVIOUS DISCUSSION... Weak showers and a few isolated thunderstorms have popped up thus far this afternoon, almost exclusively over the higher terrain. These storms are in an environment of little instability and thus have remained on the weak side. Also, given the relatively dry boundary layer, not expecting much precip to reach the ground. Have seen some wind gusts up around 30 mph in vicinity, but those are likely more related to the pressure gradient than outflow. These storms should weaken by mid-evening. The main show arrives tonight as an upper-level disturbance currently over the Gulf of California rotates around a four corners high and into Utah as an MCV late tonight. This disturbance will accompany an air mass of 1"+ PWAT as it approaches our area. Should see radar returns with this feature moving into southern Utah by the late overnight and early morning hours. Models then continue to carry it along the I-15 corridor through central Utah and eventually northern Utah / southwest Wyoming by the end of the day. While convection-allowing models show noteworthy precip amounts, BUFR soundings show limited convective potential. Indeed, taking a look at other fields it appears the the mechanism for precip creation tomorrow morning will be mesoscale to synoptic level forcing. Thus, expecting more of a widespread light to moderate rain event for roughly the southern third of the forecast area instead of isolated to scattered heavy precip. That said, expect water to be flowing in dry washes and slot canyons in the morning, and those with outdoor plans should be ready for thunderstorms at an atypical time of day (overnight and morning). Any burn scars will also be susceptible to flash flooding. For central & northern Utah, the flash flooding potential tomorrow will hinge on timing of the upper-level disturbance and associated clearing behind it. While the overall environment is not favorable for flash flooding due to lack of instability and cloud cover, a quicker push of the disturbance could give enough clearing for destabilization and better storm development. Of course, any storms that do form in the wake will be fighting weak subsidence aloft and a more stable mid-level profile in the wake of the disturbance. Given this uncertainty, have held off on a flash flood watch for now in coordination with WPC and neighboring offices. We will continue to monitor today and tonight for any potential changes and better timing certainty from convection allowing models. While Thursday will not feature a strong upper-level trigger like Wednesday, the flash flood threat is likely greater due to better instability to support stronger individual storms while PWAT remains somewhat elevated. A drying and warming trend develops from Friday into the weekend. The one exception is far southern Utah where elevated moisture will remain and promote afternoon/evening thunderstorms. The ridge aloft will remain over the Four Corners region through Monday then get squashed southward on Tuesday as a short wave trough moves across the Northern Rockies. So for the three days that the high remains near the Four Corners there will be an increasing trend in moisture moving northward across the region. Terrain induced thunderstorms are expected each afternoon into the evening from the Arizona border north northeastward to the Uintas that will likely drift into the adjacent eastern valleys by evening. As mentioned earlier, the short wave moving eastward across the Northern Rockies on Tuesday should help bring moisture northward which is more in line with the operational EC and the EC ensemble mean. Therefore, have leaned toward the higher PoP solution, rather than the GFS which brings in a westerly flow a little faster and dries the region out. && .AVIATION... Winds will shift back to southeast by 03Z. Southerly winds will ramp up after 08Z with gusts over 20 kts expected. Scattered thunderstorms, initially with gusty winds over 30 kts are possible after 21Z Wednesday. && .FIRE WEATHER... A change in weather is coming tonight as monsoon moisture rapidly spreads into the area. Temperatures will run up to 10 degrees below normal tomorrow, while south winds will remain elevated tonight into tomorrow. Most higher-elevation areas should see wetting rains with thunderstorms, and there is a chance for the valleys as well. Another round of wetting afternoon thunderstorms is expected Thursday for all districts except far northwest Utah. Warming and drying then arrives for the weekend into next week, with lingering thunderstorm chances over higher terrain. && .SLC WATCHES/WARNINGS/ADVISORIES... UT...None. WY...None. && $$ PUBLIC...10/Van Cleave/Struthwolf AVIATION...Struthwolf FIRE WEATHER...Van Cleave For more information from NOAA's National Weather Service visit... http://weather.gov/saltlakecity For frequently asked questions about the Area Forecast Discussion visit... http://weather.gov/saltlakecity/general/afd_faqs.php