Area Forecast Discussion
Issued by NWS Phoenix, AZ

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FXUS65 KPSR 220536
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
1036 PM MST Thu Mar 21 2024

.UPDATE...Updated 06Z Aviation Discussion.

&&

.SYNOPSIS...
Tranquil weather conditions with above normal temperatures will
continue into the first half of the weekend. However, a seasonally
strong disturbance will sweep through the Southwest early next week
causing temperatures to abruptly fall into a below normal range.
Along with modest rainfall chances, very breezy conditions will
become common over the weekend before the storm system exits the
region by the middle of next week.

&&

.DISCUSSION...
Western Conus blocking has finally relaxed with a pesky southern
stream closed low ejecting into eastern New Mexico while northern
stream ridging retrogrades poleward in response to invigorated East
Pacific energy advancing towards the West Coast. Over the next 38
hours, the resulting pattern will be locally highlighted by
temporary shortwave ridging and H5 heights peaking on Friday near
576dm. This will result in widespread minor HeatRisk across most
lower elevation communities with temperatures 4F-8F above normal. In
fact, recent NBM probabilities suggest some areas in SE California
having a 30-50% chance of even reaching the first 90F of the season.

As early as Saturday, pronounced troughing and height falls will
migrate into southern California with a 160+kt jet streak easily
dislodging any remaining ridging aloft. While the most aggressive
cooling will lag until Sunday when H5 heights fall below 550dm, the
first vestiges of temperatures retreating from above normal levels
will ease into the forecast area Saturday. However, with robust
height falls focused into SE California during a preferred time of
peak heating, strong winds descending from southern California
mountains will be the greatest concern with this system. Models
suggest solid 40kt winds in the H9-H8 layer juxtaposed with a cold
front crossing the coastal range and deepening windward marine layer
very favorable for downsloping and mountain rotor activity. Wind
advisories will very likely be needed for portions of the western
CWA with the greatest uncertainties being the inland expanse of the
stronger winds and temporal duration into the evening/overnight
hours.

By Sunday morning, models are in good agreement that a lead
shortwave will already be exiting the forecast area with the trough
axis continuing to intensify, awaiting a secondary vorticity center
digging through the western potion of the cyclonic flow. While the
depth of the trough and embedded vorticity forced ascent would argue
for excellent rainfall chances, total available moisture may be at a
premium with forecasts indicating mixing ratios less than 5 g/kg and
PWATs barely edging over 0.50". Consensus among ensembles indicates
an initial round of showers along a steep cold front combined with
some isentropic and orographic influences through SE Arizona, but
likely missing the majority of the forecast area. The aforementioned
secondary vorticity center will provide the next opportunity for
better ascent, however this feature and associated cold core may be
mistimed, quickly progressing through the region during the
overnight hours. GEFS members remain unenthusiastic regarding
rainfall at lower elevations with little more than a tenth per the
ensemble mean across south-central AZ, and essentially nothing of SW
AZ/SE CA. Higher terrain areas of northern and eastern AZ should
expect to see the greatest amounts, albeit likely not much over
0.50" totals.

After this trough exits into the plains by the middle of next week,
ensembles depict some form of ridging building back across the West
Coast and spreading into the Southwest. However, there remains a
decent amount of uncertainty with respect to the amplification of
this ridge, thus affecting the magnitude of the associated warming
trend. Many NAEFS members also suggest a quick moving shortwave
sweeping though the Great Basin and dampening the ridge amplitude
during the latter half of the week. While rainfall with any
subsequent shortwave would not be expected given the forecast track
well north of the area, another round of gusty spring winds would be
possible.

&&

.AVIATION...Updated at 0530Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
No aviation weather concerns are anticipated through the TAF
period under mostly clear skies overnight. Winds have calmed this
evening and either gone VRB or begun to switch to their typical
E`rly drainage directions. Wind speeds will remain light,
generally aob 8 kt sustained, through the period. High clouds
(mainly aoa 20 kft) will begin to build in tomorrow morning,
becoming SCT to at times BKN by tomorrow afternoon.

Southeast California/Southwest Arizona including KIPL and KBLH:
No aviation weather concerns are anticipated through the TAF
period under increasing high clouds. Winds will remain light, with
speeds mainly aob 10 kts sustained, though gusts into the mid-to-
upper teens are expected tomorrow afternoon at KBLH and tomorrow
evening at KIPL. Directions will generally favor S to SW at KBLH
and SW to W at KIPL. High clouds aoa 20 kft will begin to build
over the region by early tomorrow morning, becoming SCT to at
times BKN by late morning.

&&

.FIRE WEATHER...
Dry and tranquil conditions are expected through the rest of the
week. Above normal temperatures will continue during this period,
prior to a cooling trend going through the weekend and into early
next week. MinRH values over the next few days will dry to generally
around 15-20% for the lower deserts and 25-35% for the higher
terrain of south-central Arizona. Diurnal wind patterns are expected
for the next couple of days, prior to breezy to windy conditions
developing on Saturday ahead of another weather system that will
increase rain chances across south-central Arizona late this weekend
and into early next week.

&&

.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...None.
CA...None.
&&

$$

DISCUSSION...18/Young
AVIATION...Whittock
FIRE WEATHER...Young


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