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J. Brent Bower
and
Dale R. Durran

Abstract

Wind profiler data from Lay Creek, Colorado, along with stability data from the Lander and Grand Junction rawinsonde observations, were examined in an attempt to link various parameters in the upstream flow to the onset of strong downslope winds in Boulder. Some correlation was found between the occurrence of high surface winds at Boulder and the upstream wind direction, upper tropospheric wind shear and the vertical phase shift across the troposphere. However, these parameters alone were not able to distinguish between windstorm and nonwindstorm events. It is likely that the remaining ambiguity could be eliminated with information on the location and strength of inversions in the upstream flow.

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Mary C. Erickson
,
J. Brent Bower
,
Valery J. Dagostaro
,
J. Paul Dallavalle
,
Eli Jacks
,
John S. Jensenius Jr.
, and
James C. Su

Abstract

In the spring of 1990, the National Meteorological Center (NMC) tested several modifications to the Regional Analysis and Forecast System (RAFS). In order to compare the proposed version of the RAFS to the current operational RAFS, NMC ran both systems in parallel for a 4-week period. Simultaneously, the Techniques Development Laboratory (TDL) applied the operational RAFS-based Model Output Statistics (MOS) equations to output from both the operational and proposed (parallel) versions of the Nested Grid Model (NGM) to generate two sets of MOS forecasts. Our goal was to determine the impact of RAFS modifications on the NGM MOS forecasts. At the end of the 4-week test period, we verified both the operational and parallel NGM MOS forecasts. Virtually no differences in accuracy or skill existed between the operational and parallel MOS forecasts of max/min temperature, probability of precipitation, and surface wind. The skill of the cloud amount forecasts, however, deteriorated significantly. The NGM 300-mb relative humidity field changed markedly as a result of the RAFS modifications, and this change affected the cloud forecasts. Since the cloud cover forecasts were the only NGM MOS products adversely impacted by the new parallel RAFS, we rederived the cloud equations without the 300-mb relative humidity. These equations were implemented operationally in September 1990. When the new RAFS is implemented, we expect that the impact on the current NGM MOS guidance will be minimal.

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Eli Jacks
,
J. Brent Bower
,
Valery J. Dagostaro
,
J. Paul Dallavalle
,
Mary C. Erickson
, and
James C. Su

Abstract

In this paper, we describe the development and use of new nested grid model (NGM)-based model output statistics (MOS) guidance that has been available since 26 July 1989 for 204 stations in the contiguous United States. The new guidance, which replaced the NGM-based perfect prog package that had been operational since May 1987, consists of forecasts of max/min temperature, probability of precipitation, cloud amount, and surface wind. Guidance for all four elements is available for projections of 1 and 2 days from 0000 and 1200 UTC. The limited-area fine-mesh model (LFM)-based MOS guidance package is still available and was not affected by this change. Verification on independent data shows that NGM-based MOS and LFM-based MOS temperature forecasts are about equally accurate and that both sets of MOS guidance are clearly superior to the NGM-based perfect prog guidance. For the probability of precipitation, the NGM-based MOS guidance is consistently more skillful than the perfect prog guidance, and usually more skillful than the LFM-based MOS guidance. For cloud amount, the NGM-based MOS forecasts are more skillful than either the LFM-based MOS or the NGM-based perfect prog. Finally, the NGM-based MOS and perfect prog wind forecasts are about equally skillful, and both sets are superior to the LFM-based MOS guidance.

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Brian Henn
,
Qian Cao
,
Dennis P. Lettenmaier
,
Christopher S. Magirl
,
Clifford Mass
,
J. Brent Bower
,
Michael St. Laurent
,
Yixin Mao
, and
Sanja Perica

Abstract

The 22 March 2014 Oso landslide was one of the deadliest in U.S. history, resulting in 43 fatalities and the destruction of more than 40 structures. We examine synoptic conditions, precipitation records, and soil moisture reconstructions in the days, months, and years preceding the landslide. Atmospheric reanalysis shows a period of enhanced moisture transport to the Pacific Northwest beginning on 11 February 2014. The 21–42-day periods prior to the landslide had anomalously high precipitation; we estimate that 300–400 mm of precipitation fell at Oso in the 21 days prior to the landslide. Relative only to historical periods ending on 22 March, the return periods of these precipitation accumulations are large (25–88 yr). However, relative to the largest accumulations from any time of the year (annual maxima), return periods are more modest (2–6 yr). In addition to the 21–42 days prior to the landslide, there is a secondary maximum in the precipitation return periods for the 4 yr preceding the landslide. Reconstructed soil moisture was also anomalously high prior to the landslide, with return periods relative to the particular day that exceeded 40 yr about a week before the event.

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