The purpose of the NWP cases is to demonstrate to field forecasters how and when to use, not use, or modify NWP forecasts in the forecast process in various scenarios. They are created by Bill Bua and Stephen Jascourt (both UCAR/COMET).
During the fall and spring, NWP models are likely to exhibit their largest errors as the atmosphere undergoes significant adjustments to the rapidly changing patterns of heating and cooling. In this case, a medium-range forecast (3-5 day) during October for a city along the Gulf Coast is influenced by the prevailing longwave pattern, the progression of short waves in the westerlies, and by the possible input of moisture from a late season tropical system. Forecasters must use the available objective guidance, in combination with their own understanding of synoptic meteorology and an awareness of climatology, in order to make a successful forecast. |
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Using Medium-Range Ensemble Forecasts (MREFs) to Assess Possible Winter Weather Events |
This is a case where we test the limit of the MREF in assessing the possibility of a eastern U.S. winter weather event in early December 2002. |
Short Range Ensemble Forecasts (SREFs) and Their Use in Winter Storms: A Washington DC Area Case |
This case examines the use of the SREF forecast tools in the day 2 and day 1 time frame, for an early season winter storm situation from December 5-6, 2002 in the Washington DC area. |
| Short Range Ensemble Forecasts (SREFs) and Their Use in Winter Storms: A North Carolina Case | This case, a companion to the Washington DC case, examines the use of the SREF forecast tools in the day 2 and day 1 time frame, for a severe early season snow and ice storm situation from December 4-5, 2002 in North Carolina. |
Effect of Snow Depth Errors on 2-m Temperatures in Wisconsin, 8 January 2003 |
Here we examine a case where the Eta was too cool on 2-m temperatures in WI by as much as 9°C on the afternoon of 8 January 2003, from its 12 UTC forecast that very day. We demonstrate that the Eta model incorrect initial snow conditions were incorrect, and how the snow cover and snow depth error arose. |
| A Comparison of Diagnosed Vs. Predicted Precipitation Type in the Eta Forecast Model: 3-5 December 2002 | When the new grid-scale precipitation scheme was implemented in the Eta model on November 27, 2001, precipitation type became available as a forecast variable. This variable can be used to complement the diagnosed precipitation type based on forecasted vertical temperature and moisture profiles. In this case, the diagnosed precipitation type from the NCEP (a.k.a Baldwin/Schichtel) algorithm is compared to the predicted precipitation type in the experimental/parallel version of the 12-km Eta model for an early winter storm in the southern and eastern U.S. |
GFS T170 Grid-Scale Precipitation Bomb During Flood Event in IA |
This case discusses one occurrence of a well-known problem with the GFS; grid-scale precipitation bombs at T170L42 resolution. Anecdotal evidence suggests that these precipitation bulls'-eyes have been more frequent this warm season, particularly in the central and northern Plains and Midwest. The time-scale, spatial scale, and effect of the GFS precipitation bombs on the forecast are examined. We also discuss whether GFS forecasts that produce such features can be useful to the operational meteorologist. |
| All forecasters are familiar with occasional run-to-run changes in forecast direction that occur with medium-range (and sometimes even short-range) forecasts in the Global Forecast Model (aka AVN/MRF). This case describes two recent model "flip-flops" in a pair of time-adjacent operational MRF runs, and shows how MRF ensemble forecasts shed light on what is actually going on in the operational MRF seasons. |
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| Initial Conditions and SREF Forecasts for the 6-7 January 2002 Northeast US Snowstorm |
This case discusses the failure of the Short-Range Ensemble Forecast (SREF) system to capture a significant snowfall over the interior northern Mid-Atlantic states and New England that occured on January 6-7, 2002. While this is a winter case, the lessons learned herein are applicable to use of the SREF in all seasons. |
| Eta-12 Forecasts For Historic Lake Effect Snows In Buffalo, NY |
An examination of how the updated Eta-12 model, with its higher resolution, improved topography, and upgraded cloud and precipitation package, performed in forecasting the initiation and evolution of the first portion of the Buffalo, NY historic lake effect snow event (24-26 December 2001). |
| When Good Models Go Bad | A look at how the Eta model led to two different forecasts during the East Coast snow storm of December 2000, due to boundary-layer forcing from SST |
| How Good Data Can Lead to a Poor Model Analysis | An example of limitations in detecting and analyzing mesoscale phenomena in the Eta model |
| How Different Data Types Impact the Eta Analysis and Forecast | A discussion of how different data types impact the analysis and forecast, based on the results of the Zapotocny et al. case study published in Weather and Forecasting (2000) |
| Provides maps to compare model-predicted and observed frequency of 24-hour and 48-hour precipitation exceeding various thresholds to serve as a reference of characteristic model behavior | |
| Spurious Grid-Scale Convection in the Eta Model: A Case Example | The AVN produces spurious precip "bombs." Now the Eta does too. Here's a detailed look at Eta model forecast fields leading up to and during an event, including forecast impact and explanation of what's going on inside the model |
| Allison Rains in Houston, TX: Were their magnitude predictable from NWP models? |
This first of three cases considers whether the volume of rain that occurred over the Houston, TX area, particularly on 8 June, was predictable using the Eta-22 and Eta-10 NCEP models |
| Tropical Storm Allison in the Southeast U.S. | An examination of the possible role of initial conditions, resolution, and precipitation processes in the predictability of Allison's movements from LA to NC from 11–14 June 2001 |
| Allison Rains in the Philadelphia, PA Area: A case of model limitations |
This last of three cases examines what the computer models showed in their forecasts of Tropical Storm Allison as it moved past the Philadelphia area on 16 – 17 June 2001 and discusses what other considerations might be helpful in the decision-making process of flash flood watches and warnings |
| A briefing on the improvement in precipitation forecasts that has, in general, continued to be noticed since the new grid-scale precipitation scheme went into effect when the new Eta-12 became operational on 27 November 2001 |
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