Area Forecast Discussion

KEY MESSAGES

* Multiple rounds of rain and storms expected northwest of Interstate 15 and in northwestern Arizona through Thursday.

* The greatest risk of flash flooding and severe storms exists over northwestern Arizona. * Dry conditions and seasonable temperatures expected this weekend and early next week.

DISCUSSION

through Tuesday.

Latest satellite imagery shows our system spinning off the CA coast in the form of a closed low. As expected, scattered showers and storms have developed across our northern/northwestern zones overnight in the left exit region of an upper-level jet streak. This activity is forecast to continue through the day across Inyo, Nye, and Esmeralda counties, though coverage and intensity will vary over time. A secondary area of convection is beginning to form across Mohave, Lincoln, and northeastern Clark counties this morning as the jet streak translates east and robust moisture advection occurs. PWATs between 1-1.25" should allow for fairly efficient rainfall within this convective regime, though quick storm motions mean any flash flooding will likely require training convection. Best chances of this are in northern Mohave County, where a Flash Flood Watch is in effect through Thursday afternoon. In addition to the flood threat, northwestern AZ will have the risk of severe thunderstorms today, and to a lesser extent on Thursday. The SPC has given this area a marginal to slight risk of severe storms, including the potential for a tornado or two, damaging winds, and hail up to 2" in diameter today. Anomalous low-level moisture and decent lapse rates will foster CAPE values between 500-1500 J/kg. Hodograph curvature is limited, but low-level wind shear is rather impressive for the area (25-35 knots in the lowest 3km based on forecast soundings). These long, straight hodographs will favor splitting supercells, with the left split carrying the greatest hail risk and the right split having the greatest tornado risk. However, abundant storm coverage/interactions may complicate or limit the severe threat. The severe risk may carry over into the overnight hours tonight if storms manage to stay in our portion of AZ.

The primary vort max of the low/trough swings through on Thursday, bringing the best chance of widespread precipitation to our region. PoPs on Thursday are 50% or more in most locations, though the southern Mojave Desert remains the greatest area of "bust" potential with PoPs 10-40%. Precipitation chances largely exit the CWA Thursday night once the shortwave trough axis clears us, but a secondary shortwave diving south will keep the larger trough overhead on Friday. This allows shower chances (20-40%) to linger, primarily in the southern Great Basin. The trough finally pushes east on Saturday, although guidance has trended slower with this. The weekend should be much drier, but if the trough continues to slow, low precipitation chances may linger in our eastern areas on Saturday.

The hydro/convective concerns are going to be the primary focus with this system, but there are other aspects worth mentioning as well. Snow levels bottom out around 8kft, but most of the precip will fall while they are 9kft+, thus limiting any winter impacts to the higher elevations of the Sierra. Gusty winds will be present as well with this storm, with widespread southerly gusts of 20-40 mph today and Thursday. Friday into Saturday, we see a shift to gusty north winds as the system pulls away. Temperatures will be near normal today, dropping to below-normal values tomorrow through Saturday. Dry and seasonable conditions expected Sunday through Tuesday as we see a lull in the storm track.

AVIATION...For Harry Reid

For the 06Z Forecast Package...Southwesterly winds persist overnight before favoring the east tomorrow morning, with speeds of 8 knots or less. Elevated southerly to southeasterly winds arrive by the afternoon, with peak gusts between 25 and 30 knots. Winds weaken after sunset and shift to the southwest overnight. Bases may lower to around 8kft on Wednesday night.

For the rest of southern Nevada, northwest Arizona and southeast California...For the 06Z Forecast Package...Southerly to southeasterly winds pick up across much of the area tomorrow afternoon, with peak gusts between 25 and 30 knots. These winds should weaken in the afternoon. Precipitation will stay out of the vicinity of most terminals, except for KBIH, where thunderstorm activity will pick up in the late morning and afternoon. The highest confidence for rainfall at the terminal is between 20Z and 24Z. Cloud bases lower to as low as 7kft tomorrow night in Owens Valley behind the thunderstorms.

SPOTTER INFORMATION STATEMENT

Spotters are encouraged to report any significant weather or impacts according to standard operating procedures.