Area Forecast Discussion
Issued by NWS Tucson, AZ

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FXUS65 KTWC 101641

Area Forecast Discussion
National Weather Service Tucson AZ
940 AM MST Sat Mar 10 2018

.SYNOPSIS...Widespread showers will occur late this afternoon into
tonight. Showers will decrease in coverage from west to east Sunday.
A few showers may linger Monday mainly across the White Mountains,
then dry conditions return Tuesday into Thursday. A stronger storm
system will likely bring more valley rain and mountain snow showers
late next week.


.DISCUSSION...IR/visible satellite imagery depicted cloudy skies
across much of southeast Arizona at this time. Some breaks in the
overcast were noted east of Tucson, but colder-topped clouds had
moved eastward into western Pima County during the past hour. Even
colder-topped clouds were noted across southern California where
some light rain was ongoing.

Based on receipt of the 10/12Z NAM12 (e.g. METTUS 12 hour PoP of 99
ending 12Z Sunday); 10/12Z GFS (e.g. MAVTUS 12 hour PoP of 51 ending
12Z Sunday); the 10/12Z Univ of AZ WRF-NAM (e.g. depicting
measurable rain within the Tucson metro area by 4-5 pm MST this
evening); and several HRRR solutions that also depict a faster onset
of showers, the official forecast was updated to reflect
considerably increased PoPs versus the previous forecast for this
afternoon into tonight.

Thus, expect scattered showers to develop mainly west of Tucson this
afternoon, then widespread showers this evening. Appears that
showers should decrease in coverage from west to east starting after
midnight as per the high resolution solutions. However, am leaning
toward a slower decrease in coverage versus these solutions, and am
favoring a downward trend in shower coverage more during the daytime
hours Sunday. Will try to resolve this timing better for the
afternoon forecast package issuance and discussion.

The upshot for this rain event: widespread coverage of mainly light
rainfall amounts, although model generated rain amounts have trended
upward, and in the case of the NAM12, upward considerably, versus 12-
24 hours ago. At this time have opted for generally less than 0.10
inch of rainfall in the valleys, and 0.10 - 0.50 inch across the
mountains. These amounts to occur by Sunday evening. Appears that
snow levels will mostly be above mountain tops, although 1-3 inches
of snow may accumulate across the highest peaks of the Catalina`s,
Mount Graham, and the White Mountains.

Information regarding the Monday into next Saturday period in the
afternoon discussion.


.AVIATION...Valid thru 11/18Z.
Scattered -SHRA developing west of KTUS early this afternoon, then
widespread -SHRA by this evening and continuing later tonight.
Scattered -SHRA mainly KTUS vicinity eastward Sunday morning. MVFR
conditions to occur especially across higher terrain. Cloud decks
mainly above 10k ft AGL early this afternoon lowering to 4k-8k ft
AGL by early this evening and continuing into Sunday morning.
Surface wind this afternoon wly/nwly 8-18 kts with gusts near 25
kts. The strongest speeds will be east of KTUS. Surface wind
variable in direction mainly less than 12 kts at other times.
Aviation discussion not updated for TAF amendments.


.FIRE WEATHER...Widespread showers will occur late this afternoon
into tonight. Showers will then decrease from west to east Sunday. A
few lingering showers will continue across the White Mountains
Sunday night and Monday. Rainfall amounts by Monday evening should
generally be from less than 0.10 inch in the valleys, and from 0.10
to 0.50 inch across the mountains. Dry conditions will prevail
Tuesday into Thursday. A stronger storm system has the potential to
bring more significant valley rain and mountain snow late next week.
20-foot winds this afternoon west to northwest 8-15 mph with gusts
near 25 mph. Winds will then mainly be less than 15 mph Sunday, then
gusty east to southeast winds especially east of Tucson Monday. Less
wind Tuesday and Wednesday followed by gusty southwest afternoon
winds Thursday and Friday.





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