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108
FXUS65 KPSR 191037
AFDPSR

Area Forecast Discussion
National Weather Service Phoenix AZ
337 AM MST Wed Jun 19 2024

.SYNOPSIS...
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After one more day of near normal temperatures, the region will
begin to heat back up as strong high pressure builds back into
the region. An Excessive Heat Warning has been issued for the
south-central Arizona lower deserts on Thursday. The evolving
pattern will also promote a significant moisture increase across
eastern Arizona also starting Thursday supporting mainly mountain
storm activity, but chances for gusty outflow winds descending
into lower desert communities. Hot temperatures with highs around
110 degrees across the lower deserts are expected through the
weekend with daily chances of showers and thunderstorms across
much of southern and central Arizona.
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&&

.DISCUSSION...
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Over the past few model runs, the incoming moisture from Mexico
starting late tonight has been looking more impressive with each
run and this should directly impact our temperatures and shower
and thunderstorm chances. For today, the region will continue to
be influenced by the upper level trough situated to our north,
keeping highs today right near normal. However, looming across
Texas and eastern New Mexico is plentiful low level moisture with
surface dew points already in the lower 60s into eastern New
Mexico. This will quickly come into play over Arizona starting
Thursday.

As the upper level trough retreats even farther to our north
tonight into Thursday, the sub-tropical high centered over the
East-central U.S. will become more elongated while shifting
westward into eastern portions of our region. This will quickly
push H5 heights across eastern Arizona to between 590-592dm on
Thursday resulting in quick boost in temperatures. The latest NBM
forecast highs for Thursday still suggest highs reaching to
between 110-114 degrees across the south-central AZ lower deserts.
The Excessive Heat Watch has been upgraded to a Warning for
Thursday only as the incoming moisture is likely to cause a decent
amount of high terrain showers and thunderstorms Thursday
afternoon and evening and lingering cloudiness well into Friday.

The tropical disturbance moving westward across Mexico later
today through early Thursday will help to enhance the east
southeasterly flow spreading westward across northern Mexico and
New Mexico before reaching southeast Arizona starting later
tonight. By sunrise Thursday morning, models show fairly strong
and moist east southeasterly low level flow spreading into south-
central Arizona. In fact, the winds are likely to pick up enough
by mid morning Thursday to bring wind gusts of 35-45 mph into
southern Gila County. By Thursday afternoon, forecast low level
mixing ratios increase to around 10-11 g/kg over the eastern
Arizona high terrain to 9 k/kg over the Phoenix area. This fairly
adequate monsoon moisture will help to support MUCAPEs of over 500
J/kg from Phoenix and areas east of Phoenix with very high
DCAPEs, especially over the lower deserts. In the upper levels, an
incoming jet streak across southern California into Utah may
provide some weak lift over Arizona, but the main driver of any
storms Thursday will be the available mid level instability. The
00Z HREF suggests modest shower and thunderstorm development
Thursday afternoon stretching from Tucson northward through Pinal
into western Gila County. Given a southerly steering flow, this
convection is not very likely to extend much toward the lower
deserts, but there is a fairly high probability of strong outflow
winds and blowing dust reaching the lower deserts, including the
Phoenix area late Thursday afternoon/early evening. The 00Z HREF
gives a 50-70% probability of 30 kt winds and a 10-20% probability
of 50 kt winds across eastern Maricopa County (including much of
Phoenix), all of Pinal County, and western Gila County. Rainfall
amounts should be fairly light with up to around a 0.5" possible
under any stronger cells.

The southeasterly moist fetch is expected to again pick up Thursday
night, lasting through Friday morning pushing dew points to near
50 degrees as far west as the CO River Valley and into the lower
60s in the Phoenix area. By Friday afternoon, this low level
moisture feed will really start to wane, but not completely shut
off as daily convection across northern Mexico will likely keep
sending bouts of low and mid level moisture northward into our
region over the weekend. Shower and thunderstorm chances Friday
are likely to be fairly similar to Thursday, but with better
chances into the south-central Arizona lower deserts. Afternoon
convection Friday will again favor higher terrain areas with a
south southwesterly steering flow in place. For Phoenix to really
see much activity, we will likely need to see decent convection
over the higher terrain of southern Maricopa and southwest Pinal
County, and/or colliding outflows from convection from the east
and southeast.

Moisture levels should continue to improve into Saturday as
ensemble mean PWATs increase to between 1.6-1.9" over much of
southwest and south-central Arizona. However, the upper level
ridge will also be strengthening and shifting right over the
region this weekend causing increasing subsidence. Forecast
confidence this weekend is fairly low due to placement of the
ridge. We will most likely have afternoon higher terrain
convection over the weekend, but it may struggle to survive into
the lower deserts. That doesn`t mean we can`t have a very good and
fairly widespread thunderstorm day with the set-up like we are
expecting this weekend, but it would take considerable colliding
outflows. For now, the NBM shows 10-20% PoPs this weekend over La
Paz and Yuma Counties, 20-30% PoPs over Maricopa County, and
30-45% PoPs over the eastern Arizona high terrain. Any convection
in this higher PWAT environment this weekend will have the
potential of bringing localized heavy rainfall.

Hotter temperatures will also spread westward Friday into the
weekend with highs topping 110 degrees across much of the
southeast California lower deserts to near 110 degrees across
southwest and south-central Arizona. The increased moisture will
also exacerbate the heat with temperatures feeling even hotter
given the increased humidities and keeping overnight lows well
above normal. In fact, overnight lows near 90 degrees in the
Phoenix area will be possible as early as Thursday night before
lows increase areawide by Saturday night with readings mostly
staying in the low to mid 80s or higher. The temperature forecast
for this weekend is of lower confidence compared to usual due to
the potential convection and cloud cover. By early next week,
ensemble guidance favors some strengthening of the sub-tropical
high with H5 heights likely rising to between 593-596dm. Moisture
levels will likely depend on the position of the high center with
some indications of the center shifting a bit more to the south
toward Mexico which may bring some drier air in from the
southwest. For now, the NBM favors this drying as it lowers PoPs
from west to east early next week. If guidance is correct and our
heights increase next week, we are nearly certain to fall under
another round of excessive heat. NBM forecast highs currently inch
toward the 115 degree mark by next Tuesday or Wednesday, but then
show signs of backing off later next week as the ridge is likely
to start to weaken.
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&&

.AVIATION...Updated at 0600Z.

South-Central Arizona including KPHX, KIWA, KSDL, and KDVT:
VFR conditions with clear skies will continue through the forecast
period. Winds will follow typical diurnal patterns with speeds
generally remaining aob 10 kts. Easterly flow will return by
09Z-10Z at the metro terminals and continue through sunrise. Winds
will then become southerly to southeasterly late tomorrow morning
before finally shifting out of the WSW in the late afternoon
around 21Z-22Z.

Southeast California/Southwest Arizona including KIPL and KBLH:
VFR conditions with clear skies will prevail through the forecast
period. KIPL could see a few more hrs of elevated westerly winds
before diminishing to around 10 kts or less overnight. Winds will
return out of the SE after sunrise. At KBLH, winds will continue
out of the south to southwest overnight and throughout the day
tomorrow with some gusts into the teens possible tomorrow
afternoon.

&&

.FIRE WEATHER...
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Near normal temperatures today will warm back into an above
normal category as the first signs of monsoon flow and moisture
return affect eastern districts late this week through early next
week. Widespread dry conditions will prevail today with minimum
afternoon humidity levels 10-15% following poor to fair overnight
recovery of 15-45%. Moisture will surge into eastern districts
Thursday morning allowing minimum RH to only fall around 25% with
fair to good overnight recovery of 40-75%. The moisture surge
will be accompanied by unusually strong easterly winds Thursday
morning through early afternoon with gusts of 35-45 mph possible.
Wind speeds will gradually weaken into the weekend with east
southeasterly winds persisting through Saturday. With the moisture
increase, scattered afternoon thunderstorms will be possible over
higher terrain areas of eastern districts and potentially into
portions of the lower deserts. Lightning with fairly low wetting
rain chances combined with erratic, gusty outflow winds may be
significant concerns for eastern districts this weekend despite
the higher RHs. Western districts should see slightly better
moisture over the weekend, but CWR will remain below 5%. High
pressure is then expected to strengthen over the region by early
next week leading to even hotter temperatures along with slightly
drier conditions.
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&&

.PSR WATCHES/WARNINGS/ADVISORIES...
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AZ...Excessive Heat Warning from 10 AM to 8 PM MST Thursday for
     AZZ534-537>555-559.

     Wind Advisory from 9 AM to 3 PM MST Thursday for AZZ560-562.

CA...None.
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&&

$$

DISCUSSION...Kuhlman
AVIATION...Salerno
FIRE WEATHER...Kuhlman