textproduct: NWS Phoenix

UPDATE

Updated Aviation Discussion.

KEY MESSAGES

- Major waterways such as the Salt and Gila rivers will continue to experience high volume flows through at least the weekend due to dam releases.

- With the exception of a slight chance for showers and thunderstorms over parts of south-central Arizona today, dry weather will be common through at least the middle of next week.

- Temperatures will warm very near daily normals through the middle of next week.

SHORT TERM /Today through Sunday/

Early morning objective analysis depicts a more progressive flow pattern across the Conus with elongated troughing over the Great Lakes/Ohio River valley, as well as extending from the northern Rockies into the lower Colorado River valley. In fact, WV imagery highlights a very distinct vorticity center descending into southern Nevada which will become a major player in the local forecast over the next 24 hours. Trends in both global and high resolution modeling point towards more rapid deepening and forced ascent as this feature propagates into south-central Arizona by this afternoon. Recent sfc dewpoints suggest higher low level moisture content than models have previously forecast, and with diurnal mixing and upslope flow, mixing ratios 6-7 g/kg ascending and saturating near the H7 layer seems attainable. As the vorticity center intensifies and becomes a closed circulation, synoptic ascent will also increase further supporting the formation of scattered showers from Phoenix north and east. While midlevel lapse rates are not particularly steep, the introduction of modest cooling aloft and moisture intrusion at the top of the boundary layer may tap 100 J/kg MLCape leading to a few isolated thunderstorms. Most recent initialized NBM POPs were higher than previous iterations, however have raised and expanded these values somewhat based on the various HRRR and REFS trends.

In the immediate wake of this vorticty center and on the western periphery of the closed circulation, higher momentum northerly winds will surge down the lower Colorado River valley this afternoon through Saturday morning. Winds speeds within the H9-H8 mixing depth do not appear any stronger than 20-30kt, and high resolution models forecast gusts over 30 mph only across ridge tops and 20-30 mph at lower elevation population centers resulting in minimal impacts. As the cutoff vorticity center travels into northern Mexico over the weekend, sharp ridging over the Great Basin will force strong subsidence and sfc pressure rises. This classic cold season pattern will shift enhanced nocturnal/morning easterly winds into south- central Arizona further drying the environment above the H9 layer with total column PWATs falling well below 0.50". Otherwise, notable H5 height rises behind the exiting wave with readings 582-585dm will support temperatures warming back very near daily normals.

LONG TERM /Monday through Thursday/

With northern stream jet energy retreating towards the Canadian border under broad ridging, several weak low amplitude features will align in the southern stream over northern Mexico during the first half of the week before large scale hemispheric pattern amplification becomes a dominating feature. Predominant anti- cyclonic flow will continue to promote surface pressure rises over the intermountain West and dry N/NE boundary layer flow. The vast majority of ensemble membership keeps any notable shortwave ascent removed from the CWA, and confidence is very good that dry conditions will prevail through at least the middle of the week. With H5 heights remaining nearly constant in a 580-584dm range, forecast numerical spread is rather narrow indicating very good confidence in a persistence type forecast and readings very near climatology. Ensemble spread grows markedly towards the end of next week as pronounced northern stream pattern amplification driven largely by an unusual early season sudden stratospheric warming (SSW) event unfolds. The perturbation of the polar vortex should cause poleward ridge retrogression while deep negative height anomalies descend equatorward into the NE Conus and Pacific NW. While operational models are consistent in focusing strong jet energy and Pacific moisture towards northern California with inconsequential shortwave energy into the Southwest, there is a non- trivial amount of NAEFS members suggesting much deeper, intense troughing moving into the local area next weekend. Given this amplification phase, continued wide ensemble spread and potentially large variations in operational models yielding low forecast confidence should be expected into next week.

AVIATION

Updated at 1750Z.

South Central Arizona including KPHX, KIWA, KSDL, and KDVT: The main aviation weather issues will be low confidence on wind directions, especially late this afternoon through this evening, and a period with low chances (10-30%) of VCSH/SHRA conditions mainly focused to the east and north of the Phoenix airspace between 00-07Z. Winds will overall favor E to NE directions, however, directions may temporarily favor NW late this afternoon into this evening (low to moderate confidence). Speeds should be fairly light through tonight, AOB 8 kts sustained. However, if virga or SHRA develops near the terminals, winds could become briefly gusty (gusts up to 20 kts) and erratic. TEMPO BKN decks between 060-090 may also develop while virga/SHRA is present in/near the Phoenix airspace. Overnight, winds above the surface will increase out of the east, causing a minor LLWS concern, but it is expected to remain below TAF thresholds. A surge of easterly winds with sustained speeds between 8-12 kts and gusts into the middle to upper teens is then expected Saturday by mid to late morning.

Southeast California/Southwest Arizona including KIPL and KBLH: Occasionally gusty NW/N winds will be the only weather issue through the next 24 hours under clear skies. Winds will favor NW to N directions through the TAF period at both terminals. Speeds have already increased this morning, with gusts between 20-25 kts possible at KBLH and 15-20 kts at KIPL through this afternoon. Speeds will relax to AOB 11 kts this evening and through early Saturday morning.

FIRE WEATHER

A few isolated showers or thunderstorms may impact higher terrain areas north and east of Phoenix today, however wetting rainfall chances are minimal. Otherwise, dry conditions will prevail across the region through at least the middle of next week. Minimum afternoon humidity levels will largely fall into a 20-30% range in western districts and 30-50% in eastern districts. This will follow fair to good overnight recovery in western districts and good to excellent levels in eastern districts. Northerly winds gusts 20-30 mph will be common in western districts this afternoon through Saturday, while a period of enhanced morning easterly winds will develop over Arizona early next week. Outside of these time frames, wind speeds will be light with limited gustiness.

PSR WATCHES/WARNINGS/ADVISORIES

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