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HMT Forecast for 27 Dec 2008

Synopsis: ESRL and NWS forecasters pretty much in agreement. Broad ridge in the east Pacific centered about 155W is creating WNW flow on the West Coast. In this flow are two imbedded short wave troughs one due to influence the HMT area on 28 Dec 06GMT and the second close on its heels moving in on the 29th/00GMT. With the ridge shifting to the east both waves have warm advection signatures in the time-height cross-sections over the HMT area. The moisture associated with these two systems have weak coupling to the subtropics over the Philippines, but IPW values in the eastern extremity are quite low ranging from 16-24mm. The moist plume in the eastern Pacific is also quite far north (above 40N) so the moist advection is coming in from the WNW.

Precipitation outlook: With the low water content, weakness of the dynamics, and speed of these waves, precipitation will be light, 24 hour totals amounting to only about 0.3-0.5 for the first one and 0.4-0.6 for the second.

Operational recommendation: Neither event qualifies for IOP criteria for amount of precipitation, even though there is a weak warm advection signature and connection to a weak plume.

Long Range Outlook: Convection over the equatorial west Pacific is shifting eastward and this may help to move the ridge over the continental western US creating troughy conditions in the east Pacific by the middle part of next week. A trough of good strength is evident in the GFS at about 145 W on Wednesday (2 Jan) and this is predicted to move steadily eastward. The southerly flow ahead of the trough taps into subtropical moisture southeast of Hawaii so a significant moist plume or river should be associated with the warm advection sector of the trough. Based on the GFS this first wave should move in on Thursday (3 Jan) and have significant precipitation (2-3 inches)with high IPW values (30-40mm). After this first system the door will be open for a couple of additional events Friday into Saturday (4-5 Jan), and the following week.

McGinley