Area Forecast Discussion
Issued by NWS Phoenix, AZ

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744 FXUS65 KPSR 182359 AFDPSR Area Forecast Discussion National Weather Service Phoenix AZ 459 PM MST Sun Apr 18 2021 .UPDATE... Updated 00Z aviation discussion. && .SYNOPSIS Isolated sprinkles and showers are expected this afternoon and evening across central Arizona as a low pressure system drifts southward through the state. An isolated thunderstorm capable of producing strong wind gusts is also possible. Above normal temperatures will resume Monday with highs climbing into the low 90s across the lower deserts. Another weather system during the middle of next week will bring a return of somewhat cooler temperatures and increased breezy conditions. && .DISCUSSION... Latest water vapor imagery reveals cyclonic flow across Arizona with drier air positioned across the Mojave Desert. RAP analysis confirms the presence of a closed low at 700 mb, which is evident on water vapor imagery across southern Maricopa and Pinal Counties. Surface observations also suggest the presence of a surface low or trough a bit further south. Given the positive tilt of the upper trough, anomalously strong northeast flow is resulting in breezy conditions across much of the forecast area. Main impact from the aforementioned low pressure system will be the potential for isolated convection across central Arizona this afternoon. Latest mesoanalysis does not indicate any organized areas of instability across the Desert Southwest, however 12z run of the HREF and GEFS suggest max SBCAPE/MUCAPE up to 250 J/kg will be sufficient for isolated showers and storms to develop this afternoon. Latest hi-res guidance including the HRRR indicates showers and isolated thunderstorms will blossom rapidly across central Arizona early this afternoon. Meanwhile, neighborhood probabilities indicate the threat could extend as far west as La Paz and western Maricopa counties as the low drifts due southward. Although dewpoints have increased considerably since this time yesterday, conditions remain relatively dry near the surface with dewpoints in the 20s and 30s. This will yield strong evaporational cooling along with areas of virga/sprinkles, and consequently an environment favorable for downburst winds. The strongest cells will be capable of producing gusts in excess of 40+ mph as well as localized blowing dust. The abundant dry air will also limit rain totals, with only a few hundredths of an inch likely in the strongest cells. Conditions are expected to improve rapidly early this evening as subsidence builds in behind the trough axis. && .PREVIOUS DISCUSSION... High temperatures will begin to climb on Monday and Tuesday as the storm system moves out of the area. Tuesday will likely be the warmest day this week with mid 90s across southeast California and low 90s for south-central Arizona. The Imperial area shows a 10-15% chance of reaching as high as 100 F according to NBM temperature probabilities. By Wednesday, yet another shortwave will begin to move in from the west. WPC cluster analysis shows that the uncertainties with this system have shrunk since yesterday, but still revolve around how far southwest the system will dig and how tight of a pressure gradient will develop over the forecast area. The ECMWF ensemble firmly supports the idea of a deeper and more elongated trough and has little overlap with the GFS ensemble, which has a weaker trough that does not dig to the southwest. The main implications this will have on the forecast will be with respect to the amount of cooling we see and how breezy conditions will become. The mostly overland trajectory of this system along with the lack of favorable dynamic support given its positive tilt will make precipitation unlikely, which ensemble guidance all agree upon. ECMWF EFI shows the potential for anomalously strong winds on Wednesday as this system approaches. The potential for strong wind combined with dry conditions may lead to heightened fire weather concerns. The trajectory of this system will likely favor the strongest winds in the Imperial area, but breezy conditions would be expected everywhere. By Thursday, temperatures look to cool into the mid to upper 80s for highs across the lower deserts as heights fall with the passage of this system. Temperatures quickly rebound going into the weekend as ensemble members show good agreement in upper level ridging returning. && .AVIATION...Updated at 2359Z. South-Central Arizona including KPHX, KIWA, KSDL, and KDVT: Isolated convective activity will continue to dissipate over the next hour with no showers expected by 01Z. Gusty N-NW winds that have frequently been sustained at 10-20 knots with gusts up to 30-35 knots will continue weakening over the next hour as well, with some variability with the winds as showers dissipate over the East Valley. Blowing dust impacts and dust devils observed near KIWA should begin to subside as well over the next hour with ceilings continuing to lift above 10-15 kft and eventually clearing after sunset. Winds will gradually become E-NE overnight at 5-10 knots with light and variable or easterly winds tomorrow morning before becoming westerly between 20-22Z. Winds should generally remain below 10 knots with only very brief gusts. Southeast California/Southwest Arizona including KIPL and KBLH: Strong northerly 15-25 knot winds with brief gusts up to 30 knots should subside below 10-15 knots by 02Z. Winds this evening and overnight will generally favor N-NW headings at KBLH and more W-NW headings at KIPL before becoming light and variable tomorrow morning across the region. Clear skies will persist through Monday afternoon with winds generally remaining below 10 knots. && .FIRE WEATHER... Wednesday through Sunday: Hot, dry and windy conditions are likely Wednesday ahead of a low pressure system. The greatest fire danger will be across southern Arizona, where MinRHs will fall into the single digits. Conditions will improve somewhat Thursday and Friday as temperatures and RHs return to seasonable values. A warming trend is then anticipated next weekend. && .SPOTTER INFORMATION STATEMENT... Spotters should follow standard reporting procedures. && .PSR WATCHES/WARNINGS/ADVISORIES... AZ...None. CA...None. && $$ DISCUSSION...Hirsch PREVIOUS DISCUSSION...Hodges AVIATION...Hopper FIRE WEATHER...Hirsch

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