Scientific Forecaster Discussion
fxus65 kpsr 191143
Area forecast discussion
National Weather Service Phoenix Arizona
443 am MST Tue Sep 19 2017
Update...updated aviation and fire weather discussions.
dry air over the southwest will persist through Wednesday as
temperatures remain near seasonal normals across southeast
California and southern Arizona. A strong and expansive area of
low pressure will develop over the western U.S. During the latter
part of the week bringing cooler temperatures, breezy conditions,
and the chance for showers and a few thunderstorms to eastern
portions of Arizona for Thursday and Friday. Well below normal
temperatures and dry conditions will move in across the region
westerly dry flow still persists across the Desert Southwest with
much of the region under clear skies. Near seasonal temperatures
will again be seen today and even on Wednesday as an expansive
low pressure trough remains situated across the northwest U.S.
Recent models runs have slowed up the progression of the upper
level jet Max diving southward along the western side of the
closed low. This will keep our flow westerly through Wednesday
with moisture remaining well south and east of our area.
Starting Wednesday night, a strong shortwave trough is shown
diving southeastward through northern California causing the
southern portion of the deep trough to dig southward into the
Great Basin region on Thursday. As this occurs, our flow will turn
more southerly allowing for at least some moisture seepage into
Southeast Arizona, but unlike the past several days models are
now more stringent with how much moisture makes into eastern
Arizona. Due to the slower progression of the low, pops for
Wednesday and early Thursday have been lowered to below 10% while
pops later Thursday into Friday have been lowered some across our
eastern zones because of the decrease in expected moisture.
Breezy to windy conditions on Thursday still look very likely as
the pressure gradient tightens in response to a deepening surface
low over Utah. Gusts Thursday afternoon should reach 30-35 mph
across portions of southeast California and to around 25 mph
across southwest and south-central Arizona. The cold frontal
passage on Thursday now looks to mainly pass through Thursday
evening into the overnight hours, so highs on Thursday have been
adjusted upward a couple degrees from previous forecasts.
Lingering rain chances are possible through Friday afternoon
across southern Gila County, but gradual drying will be taking
place on Friday behind the cold front. Temperatures across the
region on Friday will be the coolest since mid may as highs drop
down into the mid to upper 80s across the lower deserts. The upper
level trough is forecast to remain situated across the southwest
U.S. Through the weekend as a strong ridge persists across the
eastern U.S. Dry westerly flow and continued cool conditions are
in store for the region at least through the weekend and mostly
likely into early next week. We should see temperatures start
climbing back toward normals or even slightly above normals
starting the middle of next week.
Aviation... south-central Arizona including kphx, kiwa, and ksdl;
and southeast California/southwest Arizona including kipl and kblh:
No aviation concerns through the taf period under mostly clear
skies. Wind directions to mainly follow typical diurnal trends.
Wind speeds to remain mainly on the light side through early
afternoon, then increase a bit and become rather gusty through
about 02z, as low pressure begins to approach the region from the
northwest. Winds will weaken after sunset and Switch Back out of
the east by 08z at most area terminals.
Aviation discussion not updated for amended tafs.
Wednesday through sunday:
Persistent low pressure over the Great Basin will keep flow aloft
mainly out of a west to southwest direction through the entire
period. A modest increase in moisture will occur Wednesday,
bringing a slight chance of thunderstorms primarily to southern
Gila County on Thursday and Friday. Gusty west to southwesterly
winds are expected Wednesday and especially on Thursday, with the
strongest winds occurring across parts of southeast California.
Much cooler temperatures will arrive Friday behind a cold front,
falling to around 10 degrees below normal for the date. Humidities
will fall mainly into the 15-30 percent range with good overnight
Spotter information statement...
spotters should follow Standard reporting procedures.
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