Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 182221

Area Forecast Discussion
National Weather Service Tucson AZ
320 PM MST Thu May 18 2017

.SYNOPSIS...Dry conditions will prevail into next week. High
temperatures Friday will remain below seasonal normals, then much
warmer temperatures will occur by Sunday and continue into next week.


.DISCUSSION...A deep low pressure system aloft this afternoon was
centered over western Colorado, and a strong high pressure system
aloft was off the West Coast and centered near 35N/135W. Scattered
high-based and vertically-challenged cumuloform clouds with gusty
west-to-northwest winds prevailed across southeast Arizona at this
time. These clouds will dissipate this evening with the loss of
daytime heating, and winds will diminish this evening, especially
from Tucson westward into western Pima County.

However, expect some elevated wind speeds east of Tucson later
tonight as a vort lobe moves southeastward across the Four Corners
region. Winds will then diminish across eastern sections by daybreak
Friday as the mid-level trough axis moves east of the area. Any
measurable precip associated with this vort lobe will remain north
of this forecast area.

A dry northwesterly flow aloft regime will occur Friday between the
filling upper low over the central Rockies and the aforementioned
upper high off the West Coast. The 18/12Z GFS/ECMWF/CMC then depict
fairly light westerly flow aloft to prevail this weekend as a Rex
Block (i.e. high pressure centered near 35N/135W, and weak low
pressure centered near 25N/133W) becomes established over the
eastern Pacific. These systems are progged to move gradually
eastward toward the West Coast (Pacific Northwest upper high; weak
upper low west of southern California) by Tuesday. As a result, the
operational GFS/ECMWF/CMC were in excellent agreement with depicting
an amplifying upper ridge over the area Sunday into Tuesday.

Have noted that the 18/12Z GFS were similar versus previous GFS
solutions in depicting light rain amounts that may potentially occur
near the New Mexico border Sunday night and Monday. However, the
ECMWF/CMC continue to not support this scenario, with their
respective measurable accum precip fields farther east or north of
this forecast area. Believe the GFS has a bias that is too far west
with accumulated precip this time of year, and this solution is once
again discounted. Thus, have maintained dry conditions area-wide
Sunday night into Monday.

Thereafter, there were differences between the deterministic GFS/
ECMWF regarding moisture transport from the southwest and associated
with the weak upper low that is progged to be over southern
California, or northern Baja California, next Wednesday. The GFS
depicted markedly more robust moisture fields versus the
considerably drier ECMWF. Assuming the GFS to be reality, a few
hundredths of an inch of rainfall amounts would occur southeast of
Tucson next Wednesday. Have opted to maintain dry conditions next
Wednesday with the belief that any measurable precip will be east of
this forecast area. Mostly clear skies should occur next Thursday
under dry westerly flow aloft.

High temperatures Friday will be quite similar to temperatures
achieved this afternoon, and will remain about 6-10 degrees below
normal. A pronounced warming trend is on tap this weekend, with high
temperatures Sunday to approach the century mark for the Tucson
metro area. However, high temperatures Monday (and perhaps Tuesday)
appear to be even warmer/hotter, and thus Monday and/or Tuesday
appears to be a better chance of reaching or exceeding 100 degrees
officially for Tucson. A very minor moderation in daytime
temperatures is then forecast to occur Wednesday into next Thursday.


.AVIATION...Valid thru 20/00Z.
SCT clouds around 8k-12k ft AGL into early this evening then SKC
through Friday afternoon. Surface wind thru this evening wly/nwly 12-
22 kts with gusts to near 35 kts. Surface wind diminishing to less
than 12 kts most locales by 19/09Z and continuing into Friday
morning. Expect occasional wly/nwly gusts to 20 kts Friday
afternoon. Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Dry conditions will prevail into next week. High
temperatures Friday will remain several degrees below normal, then
much warmer temperatures will occur by Sunday and continue during
the upcoming week. Gusty west to northwest winds will diminish this
evening, then markedly less wind is on tap Friday into next week.
However, expect occasional afternoon gusts due to strong daytime






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