2006-12-19 Daily Wx Discussion & Forecast
Cold, dry weather persisted across the ARB during the last 24 h , and it will continue through tomorrow (Wed.) under a ridge aloft. Far to the SE, the 500 mb cut-off cyclone will drift slowly Eward into NM by tomorrow morning and then intensify. This system will likely generate a potpourri of significant winter weather across CO, WY, KS, and NE tomorrow into Thursday… which could have a bearing on deployment for a possible IOP in the ARB on Thursday (see below).
The models are still on track regarding a transient shortwave trough breaking through the ridge and impacting the ARB between Thursday morning and early Friday. This Eward-moving wave is evident in the IR satellite imagery near 38N/142W. Newer model runs have increased the intensity of this system somewhat as it makes landfall on Thursday, although they have been consistent with its progressive character. The 12Z GFS shows the development of a long, narrow PW plume (i.e., atmospheric river) extending from the eastern Pacific to the northern/central CA coast on Thursday. Core values w/in the plume are forecast to exceed 3 cm just offshore of CA at that time. Meanwhile, HPC’s 850-700mb moisture flux product (based on the 12Z NAM run) portrays moderate normalized flux anomalies of ~+1.5-sigma over CA on Thursday. Finally, the 700 mb flow will become moderately strong (~40-50kts) and directed from the WSW during the wave passage… favorable for orographic precip enhancement. All in all, this system is shaping up to be a moderate event, albeit not long-lived (~12-15 h duration), with ~1.0-1.75” of precip expected across the ARB. As it looks now, a reasonably sharp cold fropa will occur in the 00-03Z Friday time period, with relatively short-lived postfrontal orographics due to quickly rebounding temperatures and heights aloft on Friday. Snow levels will ascend to near 8kft prior to fropa late Thursday, then crash to roughly half that altitude in the cold sector. This storm may be worthy of an IOP if deployment of personnel from Boulder, CO is feasible, although the storm will probably not have far-reaching hydrologic consequences.
During the upcoming weekend , a low-amplitude ridge will likely be situated over CA. Some zonal shortwave energy should impact the West Coast, primary in the Pacific NW… but perhaps as far S as N CA. Another moderate, progressive shortwave is currently progged to impact the ARB on Mon/Tue of next week. Thereafter, low-amplitude ridging is expected to return for a couple of days, with more shortwave energy impacting the Pacific NW.
Paul Neiman