Introduction
The winter storm that moved across Southeast Arizona at the end of February, 2003, provided a forecast challenge due to its duration. Here was a situation where the initial forecast and lead-times were quite accurate, but where we subsequently fell into the trap of trying to end the weather too quickly. After an initial 6 to 12 inches in the mountains of Southeast Arizona which verified a series of well advertised advisories and warnings, residual conditions combined with a pair of follow on impulses to bring another two separate periods of advisory level snows, with additional accumulations ranging from 6 to 16 inches. We shall look at these initial conditions and subsequent forecast indicators using the Weather Event Simulator (WES).
Discussion
On February 24, a
storm with a typical genesis in the north Pacific dug (Figure
1) moved down the coast of California before swinging through central
Arizona on Tuesday the 25th and Wednesday the 26th. The GFS seemed to have
a fair handle on things early on, advertising our best chance of precipitation
Tuesday and Tuesday night with the main 500 Mb low and vigorous short wave
(Figure 2). This turned out to be about 6 hours early,
but had the right idea. A key twist to this storm was also hinted at by the
GFS: a pair of follow-on impulses would carve into the mean trough position
and through the area Wednesday night and Friday (Figure
3).
By Monday afternoon on the 24th, it was clear that as the main portion of
the storm tracked through Arizona, the best forcing would come together behind
initial strong upslope conditions on the southern White mountains, which are
part of the east-central Arizona mountains in the northern part of our CWA.
The previously issued Winter Storm Watch was upgraded to a Heavy Snow Warning
for that area, while the remainder of southeast Arizona were changed to a
Snow Advisory above 6000 ft. After fighting through initial dry-slotting early
Tuesday (Figure 4), the afternoon saw showers spread
across the area with isolated thunder and initial snow levels around 7500
ft. Precipitable water values below 0.5 of an inch in the dry slot quickly
moistened to around 0.7 of an inch. Our well advertised winter storm products
did well overnight, with totals of 12 inches at Hannagan Meadow (200 miles
NE of Tucson in the southern White Mountains) verifying the warning, and 6
to 8 inches at Mt. Lemmon ( in the Catalinas Mountains just north of Tucson)
and Mt. Graham (roughly mid way between the previously mentioned points) verifying
advisories.
There was enough of a break Wednesday morning for solar insolation to help
us climb to convective temperatures given the residual moisture that remained
in the lower levels and boundary layer. This combined with a follow-on impulse
directly across south central Arizona (Figure 5)
to give Mt Lemmon another 6 to 8 inches of snow Wednesday and Wednesday night.
In addition, a rather deep marine later, as evident in the San Diego sounding,
(Figure 6) remained upstream and was tapped to actually
reinforce the recycled moisture for this, and the subsequent third impulse.
This third impulse carved through Thursday night and Friday. It was a little
stronger than the previous disturbance, but had a track a little farther north
(Figure 7). Both Mt. Lemmon and Hannagan meadow reported
an additional 8 inches.
Summary
This storm was well handled on the front end. There was good lead time with a Special Weather Statement that led into a Winter Storm Watch and subsequent Warnings/Advisories. Hints at subsequent weather after the initial and most significant portion of the storm were not as well keyed into as additional accumulations were under forecast. This is a good example of how a well handled weather situation on the front end can succumb to a general tendency of moving systems out of the CWA too quickly, and not tuning in sufficiently to the secondary and tertiary system. This case was subsequently selected as a training scenario to help prepare for winter weather this season. WES enabled this to be very good learning experience.
Acknowledgement
Thanks to all of the forecasters at WFO Tucson, especially Chris Rasmussen,
for their valuable insights into this complicated WES case.
