Weather Summary for the 1995 NOAA/NIST Spectroradiometer=20 Intercomparison June 18-22, 1995 Prelude The period of the NOAA/NIST Spectroradiometer Intercomparison=20 was preceded by an unseasonably cool and wet spring along the Front Range= =20 of Colorado. Following an unusually warm and dry winter, the equivalent of= =20 Boulder's expected annual rainfall (18 in.) fell from mid-April to mid- Jun= e. =20 This left the ground saturated and streams swelled. Fortunately, because= =20 Table Mountain Calibration Facility is situated high above the flood plain = and=20 the soil there is sandy and highly permeable to water, it was not severely= =20 affected by the anomalously wet conditions. =20 The pre-intercomparison period--June-15-17 The tropospheric wave pattern over North America during the three=20 days before the intercomparison was dominated by an "omega block," where a= =20 sharp ridge is flanked to the east and west by deep troughs (Fig. 1). It w= as part=20 of a persistent global wave pattern over the northern hemisphere that was= =20 also responsible for an extended hot and dry period in Ireland! Over North= =20 America, the narrow central ridge extended from Texas to southern Canada. = =20 The eastern component of this pattern was an extension of a deep trough tha= t=20 had been entrenched over northern Quebec for several months. On the=20 morning of June 16, the center of the cut off low in the western U. S. had= =20 reached as far south as southern California, and by the next morning it had= =20 drifted east into extreme southern Nevada and had weakened. At that time= =20 the low was stacked vertically above 850 mb through the depth of the=20 troposphere with nearly concentric height and vorticity isopleths at 500 mb= . =20 These characteristics indicated that it was a slow-moving and nondeveloping= =20 or decaying system. =20 As the closed low moved into Nevada, the axis of the blocking ridge=20 drifted eastward to Iowa. With this, the mid- to upper-tropospheric flow o= ver=20 the Front Range backed from southwesterly to southerly, which is conducive= =20 to strong thunderstorm activity there. By late afternoon on June 17, the d= ay=20 before the beginning of the experiment, the low over Nevada had weakened=20 to the point where it opened and was absorbed by a stronger trough moving= =20 on to the northwest coast. As the decaying trough was being assimilated, i= t=20 assumed the form of a shortwave in the southwesterly flow of the=20 approaching trough. During the afternoon of June 17, the axis of maximum= =20 vorticity advection associated with this shortwave was over New Mexico. =20 Strong thunderstorms were triggered that evening over the Front Range as=20 this perturbation propagated north over Colorado. =20 Weather patterns during the intercomparison =09June 18 Considering the unusually wet antecedent conditions, the weather=20 during the first three days of the intercomparison was fortuitously good. = By=20 the morning of June 18, the first day of spectral scans, both middle-level= =20 troughs of the omega block had propagated north. In the east, the southern= =20 periphery of the trough over eastern Canada withdrew as far north as New=20 England, but also left a weak "orphan" closed low behind in the southeast U= .=20 S. In the west, the center of the approaching tough was still off the nort= hwest=20 U. S. coast, and the maximum vorticity of the remnant short wave, that=20 triggered thunderstorms over the Front Range during the night before, was= =20 over Wyoming. Thus, on the afternoon of June 18, the northern Front Range= =20 was in descending motion on the backside of the short wave. The Denver=20 morning sounding showed significant cooling and drying between the surface= =20 and 500 mb on the back side of this wave. This, combined with the=20 maintenance of a strong inversion around 500 mb by subsidence, greatly=20 diminished the potential for thunderstorms over the Front Range. The only= =20 storms to occur in the region on June 18 were on the eastern plains of=20 Colorado along a long weak north-south oriented cold front that emanated=20 from a surface low in Saskatchewan. That boundary was instrumental in=20 keeping the high surface moisture on the eastern plains, well to the east o= f=20 the Front Range. =20 =09June 19 Over the next 24 hours the center of the approaching trough moved=20 onshore into Oregon. Its slow eastward progress acted in concert with the= =20 propagation of the short wave into Canada to allow high pressure to envelop= =20 eastern Colorado. Also, like the day before, there was a large surface moi= sture=20 gradient between the Front Range and the eastern plains. The circulation= =20 around a small surface low analyzed in the late afternoon over Denver=20 directed high surface moisture from the east to the northeastern plains of= =20 Colorado, the Nebraska panhandle, and southern Wyoming. In the Denver=20 area, continued dry conditions in the lower half of the troposphere, and th= e=20 strengthening of the inversion between 400 and 500 mb reduced the potential= =20 for thunderstorms. The middle-level inversion was fortified through a=20 combination of cooling by dry adiabatic lifting at the top of the daytime= =20 boundary layer (the base of the inversion) and warming above 500 mb by=20 subsidence. The few isolated thunderstorms that occurred that evening were= =20 over northeastern Colorado and over the Nebraska panhandle where the=20 greatest low-level moisture advection was focused. =20 =09June 20 The weather near Boulder on June 20 was similar to that of the=20 previous day. The trough over the western U. S. was weakening and became= =20 pinched between the sharp ridge to the east and another approaching closed= =20 low south of Alaska. This pincer action forced the western U. S. trough to= =20 elongate along its north-south axis. Concurrently, the large surface high = that=20 had dominated the region east of the Rocky Mountains for the past three day= s=20 began to weaken as low pressure approached from the west. However,=20 northeastern Colorado remained far from the mid-tropospheric low, now=20 centered over Idaho. Along the Front Range the surface conditions were hot= =20 and dry with high temperatures reaching the low 90s and dew point=20 temperatures in the 40s. Also, like the day before, there was a sharp mois= ture=20 gradient across eastern Colorado. The surface pressure pattern still=20 maintained the small low pressure center over the Denver Metropolitan area,= =20 but with no discernible frontal boundaries. Southeasterly flow within the= =20 northeast quadrant of this low continued to funnel warm moist air from the= =20 High Plains north of Denver into extreme northeastern Colorado and=20 southern Wyoming. The subsidence inversion above 500 mb decayed during=20 the day, but extreme dryness below 500 mb stifled convection over Denver. = =20 Again, the strongest thunderstorm activity that evening occurred to the nor= th=20 of the Denver/Boulder area where the advection of surface moisture was=20 greatest. =20 =09June 21 Over the first three days of the intercomparison (June 18-20), the=20 trough over Quebec moved out and was replaced by an intensifying closed=20 low that emerged from the Arctic. This acted to reform the omega-block=20 pattern, although much farther north than before the intercomparison. On= =20 the evening of June 20, the sharp ridge between the two closed lows extende= d=20 deep into north central Canada, with its axis skirting the western shore of= =20 Hudson Bay. This ridge prevented the weak trough over the western U.=CAS.= =20 from propagating eastward. During the day on June 21, a short-wave rotatin= g=20 around the western U. S. trough passed over Colorado. This appears to have= =20 intensified the surface low in eastern Colorado, which had drifted slightly= =20 south over the past day, causing surface easterlies and moisture advection = to=20 increase across the northeastern part of the state. Still, however, this s= urface=20 low had no associated boundaries; the nearest surface front on the east sid= e of=20 the Rocky Mountains was well to the north in central South Dakota and=20 northern Wyoming. With the disappearance of the middle-level subsidence=20 inversion and the increase in low-level moisture over the northern Front=20 Range, the potential for afternoon thunderstorms increased. For the first= =20 time in four days, the lifted index1, estimated for the time of maximum=20 heating at Denver, was negative (-4=A1C), which indicated a good chance for= =20 afternoon thunderstorms. By early afternoon, convective storms broke out= =20 along the Front Range and made for the first cloudy afternoon of the=20 intercomparison. =20 =09June 22 By the morning of June 22, the trough over the western U. S. was no=20 longer closed and was weakening as a new low approached North America=20 from the Pacific Ocean. However, the western U. S. trough would still have= =20 an influence on Colorado's weather. On the morning of June 22 the its axis= =20 passed through Idaho Utah, and Arizona, and during the next 24 hours it=20 moved into western Colorado. The local weather on June 22 was also=20 inherently affected by the cold front, which on the day before was in South= =20 Dakota and Wyoming. By the morning of June 22 it had penetrated into=20 northeastern Colorado, having been fortified by outflow from nocturnal=20 MCSs in the Dakotas. The surface low that was present in eastern Colorado= =20 the day before, persisted as a frontal-wave, and continued to pull very moi= st=20 air from the High Plains into the northeastern part of the state. In the e= arly=20 afternoon, severe thunderstorms broke out in northeastern Colorado and=20 western Nebraska. That evening, with the surface frontal boundary on which= =20 to focus, and the upper-level support of the approaching trough, these stor= ms=20 developed into two MCSs over Nebraska, eastern Colorado, and western=20 Kansas. =20 Fig. 1.=09500-mb height (solid) and temperature (dashed) analysis for 1200= =20 UTC, June 15, 1995. Station plots include winds (full barb is 5 ms-1, flag= 25 ms- 1), and, clockwise from the upper left, temperature (=A1C), the height of t= he 500- mb surface (dm), the 12-hour height change in dm, and the dew point=20 depression (=A1C). =20 Footnote: 1 The lifted index is the difference in temperature at 500 mb between the e= nvironment and that=20 of a surface parcel lifted along a dry adiabat to the lifting condensation = level, and then along a=20 moist adiabat to the 500 mb level.