textproduct: NWS Phoenix

UPDATE...Updated 18z Aviation Discussion

KEY MESSAGES

- Temperatures up to 8 degrees above normal will cool closer to the seasonal average by the end of the week allowing moderate HeatRisk to be reduced towards a minor category.

- A slight chance of storms will return this afternoon, but focused exclusively near terrain features in Arizona.

- An approaching east Pacific weather disturbance late this week will dry out the region, while bringing increasingly breezy conditions and cooling temperatures.

SHORT TERM /Today through Thursday/

Early morning WV imagery depicts a very complicated flow pattern over the SW Conus featuring an anti-cyclone retrograding across Nevada while a series of weak low pressure circulations extend along the international border westward off the California coast. Lingering moisture in the form of 6-10 g/kg mixing ratios have combined with modest ascent the past few days resulting in terrain driven thunderstorms and numerous outflow boundaries. The influence of increasing westerly flow associated with consolidating low pressure off the California coast over the next 48 hours will slowly erode boundary layer moisture while a more confluent, subsident regime eliminates ascent mechanisms.

Sufficient moisture will linger across south-central Arizona today allowing terrain induced upslope to support some isolated storms despite an increasingly hostile synoptic environment. Given this underlying lack of support, any storms should become very short lived and tied to their initiation points, however DCape values in excess of 1500 J/kg may support some well defined outflow boundaries before subsidence and MLCin overwhelms further storm formation. By Thursday, activity should become relegated to the White Mountains of eastern Arizona, only potentially clipping far eastern Gila County. Otherwise, the increasing westerly flow and introduction of jet energy will promote gusty afternoon/evening winds, particularly in the traditional downslope areas of southeast California. The change in the pattern will also allow H5 heights to gradually fall below 588dm, and forecast confidence is very good that temperatures will retreat several degrees and fall much closer to the daily normals by Thursday.

LONG TERM /Friday through Tuesday/

There is excellent agreement among ensemble membership that more pronounced East Pacific troughing will eject into the Great Basin resulting in strong zonal flow across the forecast area. This flow pattern will completely scour away moisture such that mixing ratios fall closer to 2-3 g/kg eliminating all chances for precipitation. While H5 heights will not drop markedly, subtle cooling along with the introduction of this much drier airmass will result in temperatures falling to near normal levels over the weekend. The initial round of modest height falls combined with seasonably deep mixing depths will promote repeated chances of gusty afternoon winds late in the week. Juxtaposed with the incoming dry airmass and receptive, dessicated fuels, fire danger will become quite elevated. Ensemble spreads grows somewhat towards the middle of next week, however the majority of members indicate building subtropical ridging over the CWA after the trough passage. H5 heights should rebound over 594dm with the anti-cyclone center becoming positioned directly over Arizona. Some of the more aggressive modeling in the upper quartile of the model distribution even suggests heights breaching 597dm by the middle of the week which would equate to lower elevation temperatures reaching 115F yielding expansive major HeatRisk.

AVIATION

Updated at 1702Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: Another rough of gusty afternoon winds will be the main aviation impact through the TAF period. Westerly winds will dominant again across the metro mid morning before becoming gusty during the afternoon with gusts frequently in the 20-25 kt range. This evening can expect periods of southerly winds turning NNE by tomorrow morning. FEW mid to high clouds will be common throughout the period.

Southeast California/Southwest Arizona including KIPL and KBLH: The main aviation concern will be gusty winds that will continue through the afternoon/evening under generally clear skies. At KIPL, winds will generally prevail out of the west while at KBLH, winds will fluctuate out of the south to southwest. Afternoon/early evening gusts upwards of 25 kts will be common at KBLH and upwards of 25-30 kts at KIPL.

FIRE WEATHER

A pronounced drying trend will sweep across the region the remainder of the week with terrain induced showers/storms becoming completely eliminated from the forecast by the end of the week. A few thunderstorms may form over higher terrain features this afternoon, however chances of wetting rainfall will be remote with only limited areas of gusty outflow winds. Despite temperatures cooling closer to normal later this week, a much drier airmass will allow minimum humidity levels primarily in a 10-20% range to deteriorate into widespread single digits over the weekend and early next week. Correspondingly, overnight recovery will deteriorate into a poor to fair 15-40% range. Winds will frequently become gusty the remainder of the week yielding a widespread elevated fire danger, however at this time, speeds appear under critical thresholds.

PSR WATCHES/WARNINGS/ADVISORIES

AZ...None. CA...None.