OCR Text |
Show TABLE 9 Steamboat Spring s Long-Term Ave rage Monthly Precipitation Nov Month Pr e cipitation 1. 7 5 Dec Jan 2. 41 2. 42 Feb 2. 41 Mar 2. 24 Apr Season / 2. 18 13. 41 Met event f instabi ~recip: indicat l - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - . . . J I ,ur-1 I Large scale upp e r air patterns which produc e the most persistent periods o! Aoout ~recip frequent snow are: rate01 A low to moderate amplitud e long wave ridge just off the West Tne et coast with an ill-defined long wave trough east of the Mississippi River. lation This gives over-extend e d wave length and allows short wave troughs (which arrive at 30 to 48 hour inter v als) to amplify as they approach the Continental Ra~idl chan~t Divide . (1) (2) A very high amplitude long wave rid ge extending from the centra Gulf of Alaska southward, with strong moist north-northwe sterly flow over the Park Range . Ve ry fast-moving short wave trou ghs (arriving at 20 to 30 hour intervals) intensify and decelerate slightly as they begin to curve more easterly at th e latitud e of th e Park Ran ge . The s e two patterns can result in very large snowfalls du e to the duration of precipitation conditions. -1 The highest precipitation rates (frequ ently e xceeding 0. 1 in hr ) usually occur in the first 3 to 6 hours followin g the surface-passag e of maritime po· lar fronts. This tim e period frequ e ntly c oincid es with th e existence of 240° to 270 ° flo w above 12,000 ft. MSL but 280-310° flo w at lowe r leve ls. This ii in the portion of th e short wave wher e synoptic scale maximum upward verl· ical ve locity coincid e s with the presence of a rel a ti ve ly warm column of moist air, and thus precipitation rates are at a maximum. Cloud tops durin this frontal convergence (or post-frontal over-running) precipitation are usually above 18, 000 ft. MSL. wnich idlJ rr ahead Exlre !hem -~Cti mo ar Ml. 1 ~sse rate, re!le1 ~ass, H: Fror fount Longer periods of light e r (0. 003 to 0. 050 in hr. -l), mainly oro graphic pre· wmd cipitation usually continue after passage of the frontal convergence area wm This Ran! cloud tops lowering to bet ween 15,000 ft. MSL and 11,500 ft. MSL. A classification of prec ipit ation patterns as 1) post cold frontal over-runnini (or frontal c onve r ge nc e zone), and 2) mainly oro g raphic, can account for about 80 % of the Park Range winter precipitation (Rh ea , 1967). Pre-cold frontal and warm frontal over-running precipitation types are rare, partia_ll due to a seri es of u pstr eam northwest-sou theast ori ent ed barriers at bearin 108 Iron lnis ~rei the sea] Drh |