The 12Z GFS run from 20 August 2002 showed a precipitation maximum was to move from eastern NE through IA into IL, directly through the Davenport IA CWA. Predicted precipitation amounts per 3 hours were on the order of 2-3", resulting in a concern about needs for flash flood watches and warnings.
Evolution of the GFS precipitation field
Click here to see the precipitation forecasts at 3-hour intervals from 15 UTC 20 August to 15 UTC 24 August 2002 over the Midwest. Precipitation bulls-eyes are clearly present. The first develops over SD at 21 UTC 20 August and propagates to the upper MI peninsula by 12 UTC 21 August before weakening. It is quickly followed by a second more intense precipitation maximum which develops in southeastern SD at 06 UTC 21 August and follows the same general track as the previous "bomb". This one, however, is collocated with the wave rather than out in advance of it, and propagates with the wave east-northeastward into Ontario from upper MI by 06 UTC 22 August. The precipitation maximum and the associated low pressure area retain their strength as they leave the domain of view.
A third precipitation "bomb" develops by 18Z 22 August and tracks eastward from eastern NB to southwestern WI and weakens by 18Z 23 August. This precipitation maximum is the strongest of the three forecast over the Midwest, with precipitation amounts exceeding 2" (50.8-mm) per 3-hour period at its peak. Of the total precipitation within each of the precipitation "bombs", two-thirds or more is generated by the grid-scale precipitation scheme, rather than the convective scheme. Before the "bomb" develops, most or all of the forecast precipitation is convective.
Forecast evolution
The initial conditions from the GFS analysis were shown in the previous page discussing synoptic evolution associated with the heavy rains. Click here to review the associated graphic.
Twenty-four hours later (see graphic below), the MT low is forecast to move to the SD/MN border, with fronts extending south-southwest into the TX panhandle, and eastward into WI. The surface low is further east than in the analysis, a consequence of the model's excess grid-scale condensational heating in the overrunning area to the east of the forecast low center. At 850-hPa, the forecast errors are consistent with events at the surface, with the 850-hPa low as a closed center at 143-dam over northeast SD, compared to the analyzed depth of 144-dam over south-central SD. Because of the position errors, the southwesterly jet (35-40 kts) in advance of the system at 850-hPa is a bit further east and north than analyzed. Forecast moisture pooling near and just north of the warm-frontal surface at 700-hPa over central MN and northern WI agrees well with the analysis, along with strong southwesterly flow from OK to Lake Superior. There is lower RH, however, along the cold front advancing through the Central Plains than analyzed. At 500-hPa, a shortwave trough can be found with a vorticity maximum just slightly downwind of the surface and 850-hPa lows, with an additional somewhat weaker shortwave over upper MI, while the analysis has one vorticity maximum over east-central MN. The forecast vorticity maxima are the result of grid-scale convection, and one can track precipitation bull's-eyes from earlier in the 12z 20 August forecast and see how they result in vorticity bull's-eyes.
Graphic: 24-h GFS forecast from 12z 20 August 2002, valid 12z 21 August 2002. Compare to panel 5 in the analysis animation
By 48 hours, the forecast shortwave associated with ongoing grid-scale convection has propagated east-northeastward into Ontario and is being absorbed by a larger-scale cyclonic circulation over eastern Canada. The forecast cold front is draped southwestward from the surface low through MI, northern IL and MO, and thence to a lee-side low developing in western KS. This contrasts with the analyzed 1011-hPa surface low over Lake Huron in the analysis, with a less well-defined frontal trough extending southwestward through southern WI and across central IA to broad low pressure over central KS and south-central NE.
The forecast 850-hPa low is 141-dam northeast of Lake Huron, by at least 200-km, with a collocated surface low at 1003-hPa. and 149-dam 850-hPa low in the analysis. At 850-hPa, a lee-side low also appears in western KS, collocated with a temperature maximum in excess of 27°C. Note the upslope flow at 850-hPa from northernmost KS to ND, that does not appear in the analysis.
A 700-hPa moisture gradient appears across IA, and a secondary moisture maximum appears over NE. This contrasts with the broader 80-90%+ RH found at 700-hPa further north, over areas that received the heavy rains during the night of 21-22 August. Also note the strong push of dry air to the rear of the exiting disturbance; there is much less of a southward push in the analysis, which will have implications for the day three forecast over the lower Great Lakes and adjacent Midwestern states.
At 500-hPa, we see the exiting vorticity maximum over eastern Ontario associated with the earlier grid-scale convection, a stronger upstream ridge axis from IA northwestward into the Dakotas. A few small vorticity maxima forecast appear over the western Great Plains into NE, and a shortwave trough over MT. In contrast, the 500-hPa analysis shows the reflection of vorticity generated by convective heating associated with the heavy rains over IA, IL, and WI. This disturbance does not appear in the forecast, since the physical forcing responsible for it was missed. Other forecast vorticity centers can be found over eastern WY.
48-h GFS forecast from 12z 20 August 2002, valid 12z 22 August 2002. Compare to panel 9 in the analysis animation.
At 60 hours into the forecast, the IA precipitation bull's-eye is fully developed, as can be seen here. Its influence is beginning to be felt in the forecast fields, as marked by the red oval. At the surface, an area of lower pressure is beginning to develop at the NE/IA border north of Omaha. An 850-hPa circulation is beginning to develop in the wind field (see red barbs), with 700-hPa RH over 90% in a small collocated area and 500-hPa vorticity spinning up.
60-h GFS forecast from 12z 20 August 2002, valid 12z 22 August 2002. Compare to panel 11 in the analysis animation.
Finally, by 72 hours into the forecast, a surface wave at 1012-hPa has developed, with a significant 850-hPa low and circulation over IA, a 90% 700-hPa RH area, and 500-hPa vorticity maximum and shortwave trough at the upstream end of these features over the northeast corner of IA. These features are all notes with red ovals. An additional vorticity maximum has developed over north-central IA; this will be discussed in the next section which analyzes the IA grid-scale precipitation bomb.
72-h GFS forecast from 12z 20 August 2002, valid 12z 22 August 2002. Compare to panel 13 in the analysis animation.
